I wonder why that changed in Fantasy media in general?
If you mean for manticores, my best guess would be because in fantasy they're generally seen as more beast-like. Putting a human head on a hostile beast doesn't gel well with most people's expectations.
If you mean for sphinxes... AFAIK that change hasn't really occurred. This is very much one of those "D&D is just weird about this" things. It might have knock-on effects in other media, but I've seen plenty of actual sphinxes there, too. Pathfinder, for example, has actual sphinxes:
MtG also has actual sphinxes, and that's also WotC. They get more creative with the idea sometimes, and my absolute favorite sphinx design in MtG is Ormos:
Her design really captures the avian subtheme in a way not many sphinx designs do.
Although you have also reminded me that another one of the reasons I dislike 4e/5e winged lion sphinxes is how visually similar they are to D&D manticores. The texture of the wings and thickness of the tail are just not enough of a difference to me - I want something as thematically distinct as sphinxes and manticores usually are to be visually easy to differentiate at a glance. (Yes, I know real-world mythology manticores are pretty similar in appearance to the real-world mythology sphinxes in many cases, but if one gets the human face and not the other, then IMO sphinxes - the ones with explicitly human-like sapience - should be it.)
I feel this image does a good job of portraying a human face but being a nonsapient "beast".
I agree, and I especially like the commitment to the manticore's 3 rows of teeth. A face like this could still be called human-like, but still different enough to be obviously more feral. I think the size of the head also helps with that.