Agreed with Magician and RmDebArc, and besides that, it could also be for the challenge, for the enjoyment, or because the vision of the dev only works in such a platform. And with emulators that allow commercial use around, the developer can also publish his games to modern platforms without having to rebuild the whole game.
Libretro folks are going to take the code of any maintained emulators anyways, so people don’t really need to port it. Also didn’t follow any news from Libretro’s side, but considering Libretro’s founder’s past interactions in license changes of emulators the project benefited from, I can’t help but wonder if he/she threw a hissing fit at the Duckstation folks as well.