Assets were much simpler. The screen was ~320x240 pixel so your 2D images were a handful of pixels from a couple hundred limited colors. An artist could bang out tons of images and sprites in the amount of time it takes someone to meticulously craft, model, paint, animate, etc. a full 3D monster today. Same thing for sound and music--it was much simpler and more manageable by one or small number of people.

Look at game jams today, like the 48 hour game creation contests. A lot of stuff that comes out of those are little 2D things reminiscent of games of that early 90s era. It's because it's still pretty fast and easy to make assets for that style of game.