It is hard to find affordable housing in many metro areas.
In particular, it is hard to find unsubsidized small single family homes on small lots, and this is a big part of the lack of unsubsidized affordable housing.
Lots of this is due to land use regulation, and building codes mandating minimum square footages and other unnecessary requirements are also a barrier for small home options other than true single wide or double wide mobile homes.
For example, building codes often prohibit building dormitory, or camp ground, or boarding house, or "flop house" a.k.a. single occupancy hotel style communities of small dwellings (attached or detached) in which unit tenants or unit owners have a shared bath house or share some kitchen facilities, as opposed to having a full bath and kitchen in each unit. Exceptions can be and are granted, but doing so is expense and difficult for a low to moderate income family or a small developer trying to meet their needs.
Land use regulations intended to discourage mobile homes are a big reason that small lots, cottages, accessory dwelling units, and "tiny homes" are banned so many places, not withstanding federal laws limiting the extent that building codes and more blatant zoning laws banning mobile homes all together can be used to prohibit mobile homes.
Land use regulation also mean that as a practical matter, mobile home owners in urban and suburban areas where land is not cheap have to rent the land that their mobile homes are sited, creating an exploitive and vulnerable landlord-tenant situation, instead of a stable ownership arrangement.
The Colorado Sun has a series of related articles addressing some of these issues. The opening blurb of the first article alone says a lot and frames the question correctly in my view.
Mobile homes provide the bulk of unsubsidized, affordable housing in Colorado but their numbers are shrinking and ownership is moving from mom-and-pop to corporate.The same article goes on to elaborate that:
The business model — in which homeowners pay lot rent to park their houses on someone else’s land — exposes the immobility and economic vulnerability of tenants who can’t afford to move or live anywhere else. . . . Mobile homes provide the largest inventory of unsubsidized, affordable housing in the nation, but many began as RV parks in the 1960s and 1970s and are now old, with rundown water and electric systems and trailers that have been long past “mobile” for decades. . . . the number of parks is declining and ownership is consolidating as mom-and-pop parks sell out to large investors, which sometimes leads to displacement and redevelopment — and, in the eyes of many residents, an imbalance of power that threatens their low-cost lifestyle. More than 100,000 people live in more than 900 parks across Colorado. Those residents include many of Colorado’s working poor and immigrants who are undocumented. They have been mostly ignored for decades. Born in the post-World War II era, when quick, cheap housing was in demand among returning veterans, mobile homes endured and hit a peak in the early 1970s, according to U.S. Census data, before beginning a gradual decline in new home sales. . . . 70% of homes sold under $125,000 are mobile homes. . . . . Mobile-home residents live under the radar — literally zoned out of sight and segregated from conventional housing. . . . the top 50 owners of mobile-home parks have a combined 680,000 home sites across America, a 26% increase from 2016 to 2018, according to the Manufactured Housing Institute. Investors and private equity firms, formed by investors who directly invest in other companies, now own more than 150,000 manufactured home sites, according to a 2019 report . . . . Nationwide, an estimated 20 million Americans live in mobile-home parks. Nearly 10% of the nation’s housing stock is manufactured homes, which numbered about 8.5 million in 2018 . . . . Park owners charge residents rent for the lot on which the unit sits. The tangle of rights and responsibilities for decades has favored the park owners. . . . As of Feb. 1, about 2.5 million mobile-home residents cut a rent check to a corporate entity.
The regulatory barriers to mobile homes in land use regulation don't come from nowhere, and aren't entirely irrational. They are often found in parts of cities and towns that have bad reputations for legitimate reasons. And, in a situation where land use regulation and building codes are assigned to local government, the inclination to ban this can of land use entirely and somewhat indiscriminately is one that can be indulged in because no particular local government has an obligation moral or legal, to make sure that the poor can live somewhere, and their voters are, by somewhat inadvertent design, disproportionately long term home owners and long term renters who have stable, middle class or better lives, who look out for their own interests.
In a word, many mobile home parks are "sketchy" places.
Aesthetically, the predominant type of mobile home is ugly as sin, even though it isn't all that difficult to make nicer looking ones. They are frequently shoddily constructed, and a death trap in the face of tornados, hurricanes and floods.
More significantly, there is a tendency to concentrate mobile homes close to each other in mobile home parks, and affordable housing pretty much by design disproportionately attract people who are poor or at least have modest means, and in the adult world, people who are poor or have modest means tend to be at the bottom in terms of socio-economic class above everyone but the homeless, and on average, less education and intelligence. Mobile home parks are also notorious for having high rates of crime, high rates of child abuse and neglect, having many residents who have trouble acting in a way that is respectful of their neighbors and civil and tidy and well maintained, and high rates of substance abuse. Lots of this is a product of lack of income (i.e. poverty), but socio-economic class and the fact that people who have trouble refraining from this disfavored kind of conduct and have economically undesirable characteristic often do poorly in the work force or economy.
Notable, mobile home communities that are not viewed as a strong negative for a community, tend to be restricted to senior citizens who may have low incomes because they are retired and often only have Social Security, Medicare, meager savings and low paying part-time work to provide them with an income. But, they tend not to have the issues with socio-economic class, education, intelligence, anti-social behavior, incivility, and neglect of their homes, of the poor generally.
Similarly, people who are temporarily homeless due to a natural disaster or because they are refugees, college students, soldiers and young farm families often have low income and live in residences that are spartan, temporary, and/or lack individual bath and kitchen facilities, but like seniors tend not to have the issues with socio-economic class, education, intelligence, anti-social behavior, incivility, and neglect of their homes to nearly the same extent.