I mean specifically the current (15-20 year) increase in this genre, with the intention of showing in mainstream theatres.

Shoestrings budget films have existed pretty much as long as motion cameras have been generally available.

But these are specifically created because they are cheap (relatively) to make and sell enough tickets to get into mainstream theatres, which produces much higher profits than other genres on shoestring budgets. Note of course that all of this is the general case - there are obviously exceptions in every direction.