Why is the level design so much better than Super Mario Bros 3?

because this game was avant-garde. smb3 was a mcdonalds happy meal game.

>>11826276 (OP)
IDK, can you explain?

It's not, just depends on what you want from a game. Super Mario Bros. is more limited so the levels are more tightly designed in that sense, while Super Mario Bros. 3 has more variety and experimental levels that do interesting things. Some level gimmicks in 3 work better than others.

>>11826276 (OP)
It's just a better game in general. Better physics, better artstyle, better pacing, less filler, levels flow much better, less annoying bullshit enemies, etc