Why are 55 and over trailer parks so much nice than ones that are not?

Retired people aren't going anywhere: they are a stable population. When they move into a place, they take care of it because they expect to live there the rest of their life. They don't have expenses for raising chldren, so they can live cheaply and still budget some money for maintenance.

Younger people often don't appreciate the need to take care of things. Budgeting means nothing to them. They can not distinguish "living cheaply" from "broke". In the extreme case you see dumpsters full of broken furniture. The people buy the cheapest junk they can find because they don't expect it to last long, then they don't hesitate to break it because it was trash right from the start, then they throw it out and do it again. They often move on short notice because they have no particular reason to stay in one place. The old saying is "Three moves equals a fire." That is to say after three moves they are rebuilding their lives. I have known people who moved three or more times a year. They are NOT a stable population.