I think most people prefer magic points. Spell slots are just kind of a thing that D&D happened to have. If D&D wasn't such a cultural force in defining what an RPG even is, spell slots would be a weird quirky thing that one RPG has and everyone is like "huh that's kind of interesting" and think of D&D as "that one with the weird spell slots."

Not that spell slots are meritless. They're weird and people want magic to be weird, they give you a reason to use low-level spells, and they somewhat encourage using spells flexibly (but they also encourage over-relying on the most flexible possible spells; just memorize five copies of Polymorph). But yeah, MP is the most default resource mechanic you can get.