>>717775386 (OP)
It's because the indie 2D platformer craze of ye olden days fizzled out. People realized that it actually takes a lot of work to design pure platformers and even more work to make them successful. Unless you're Nintendo making Mario games it's not really worth it.
Metroidvanias have exploration, combat, progression, and more storytelling potential, while still incorporating some platforming elements. They can be greater than the sum of their parts rather than needing to be perfect in one specific way. They're inherently more appealing.