Resin means molds. MDF means a single (well, in big operations likely several) laser cutters. I think the MDF stuff is popular for several reasons:
1) easy to pre-paint
2) cheap basic material
3) can be made to order if needed
4) no molds, no shifting, no bulk materials
5) can flat-pack which in turn makes shipping much cheaper/easier
6) because the design is nothing more than a small file on a computer a company with a single laser cutter can have hundreds of designs with no need for factory space, molds or additional storage.
I think each has its place...and I'm a bit picky about my MDF. 4Ground is simply marvelous, but I love my Crescent Root adobe structures (and adobe would look weird unless the frame was simply mdf and you plastered it after the fact). I do find a lot of resin terrain is pretty awful (old designs, sloppy sculpts etc.) so it's rare to stumble into a nice one. As for hand-made stuff, we've already beat that horse into glue.