It's more complicated than just this, of course, and to do with the production pipeline nowadays and how it's changed, but from what I can tell: the switch to digital. Everything's lit flat because you can tinker w it in post, color grade it to oblivion easier + dependence on greenscreen/CGI etc.

yeah i feel like you can instantly tell when a show was lit in post - the lighting has a very distinct softness to it, no harsh or cast shadows, low contrast, and this weird 3D effect and with a few notable exceptions, i find recent color grading trends to be repulsive haha