just a question

Probably because RenPy is super easy to build a visual novel with. You can go from nothing to a framework of a visual novel and have it build for all desktop platforms in 20 minutes. However, tools aside, the yuri genre lends itself to prose given the relationship between the characters is the most important.

Probably because people are busier and want to have the experience of having a love life :P

But actually I think it's a combination of that and what polisummer said about how Ren'py is easy to use.