The cost of complying with health and safety regulations makes it too expensive to price rent below a certain amount even at the worst the rental situation is legally allowed to be.

There's also zoning and other issues, and subtler ones like licensing of construction trades, etc. But this seems to be a big part of the story. From last year http://www.psmag.com/navigation/politics-and-law/how-the-trailer-park-could-save-us-all-55137/

By any name, they are the largest source of unsubsidized affordable housing in the country. There are seven million manufactured homes housing 18 million people. In some counties they make up 60 percent of dwellings. Approximately one out of every 12 Floridians lives in a manufactured home. Units built since 1976, when the Department of Housing and Urban Development started regulating their construction, can last as long as site-built homes when they’re well built and maintained. Yet they cost far less: $41 per square foot versus $85 per square foot and up. At least one study, from the University of Illinois-Chicago, on trailer parks in Omaha, Nebraska, found that crime rates in mobile-home parks are the same as the rest of the community; the parks do not cause crime nearby; and that the parks appear to depress crime levels because residents own their homes. In one survey, nine out of 10 owners of manufactured homes said they were satisfied with their dwellings. They’ve found a housing option that suited their budget and needs.

Commentary: http://econlog.econlib.org/archives/2013/04/why_trailers_in.html