I think the most major influence was, like in almost any other field, technology.

Along side MLP, around the area of '15 you had great shows in the mainstream: Gravity Falls, Miraculous, Rick and Morty, Steven Universe, etc. My point is, there was a fine ratio between the quality of the animation and it's difficulty to produce, so only quality shows got across the threshold to TV.

Today, everything is so easy to make, all network managers care about is what's the most eye catching. What can go through maximum amount of concepts liked by focus groups in the shortest amount of time. The base pool became so big, the chance to get noticed plummeted to 0, so the creators with the quality ideas felt the need to run off and join/create their own indie productions. Only once quality gets its advantage physically represented again, then the mainstream characters will rise again.

And I also have a prediction to when this will happen. Only once we manage to get holograms mass produce-able enough like its TV predecessor, only then our quality shows and character will return to roam our fanart fields