It is all about the taxes. Previously, the state's department of motor vehicles was collecting annual registration fees. The local real-estate tax authorities were only taxing vacant land. So there were big differences in the taxes collected for local schools, highways, first responders, etc.
Since the 80s the most mobile thing about mobile homes has been its name, For the past thirty five years our homes have evolved from travel trailers to permenant fixtures. Many states adopted laws to retire manufactured home titles. Once the title is retired, the manufactured home is then considered part of the real-estate and taxed like any other single family residence. Each state has it own system for retiring and reactivating titles for manufactured homes.
In some states retirement is mandatory when the home and land are owned in the same names. In some states there are options available. The option you choose could effect your total annual taxes. Because the rules do differ in each state. If you have any questions it is best to contact your local or State Manufactured Housing Association