It's hard to qualify one system as objectively better than another, however, for most of the games I enjoy (platformers and shoot 'em ups), the NES delivers them in much higher quality and quantity so it is easily better to me!!! Just my two cents!!!

I also tend to prefer NES/Famicom sounds to SNES, to me SNES seems to sound more like a midi with a mediocre soundfont whereas NES sounds are more of their own thing that don't really attempt to be any one instrument (other than DPCM samples) so for the most part I feel NES games have a cleaner sound to them.

Lots of games I like on both but I could probably list 50 times or more the number of games I really enjoy on NES vs SNES.

Why does one have to be better than the other?

The answer is right there in the name of the systems:

Nintendo Entertainment System
Super Nintendo Entertainment System

Family Computer
Super Family Computer

What is that funny "s" word "Super" mean?? The dictionary has your answer:

su·per

/ˈso͞opər/

adjective

adjective: super

1. 1.

   informal

   very good or pleasant; excellent.

   "Julie was a super girl"

   synonyms:

   excellent, superb, superlative, first-class, outstanding, marvelous, magnificent, wonderful, splendid, glorious; More informalgreat, fantastic, fabulous, terrific, ace, divine, A1, wicked, cool, killer; smashing, brilliant "we had a super time at the water park"

   antonyms:

   rotten

2. 2.

   (of a manufactured product) superfine.

   "a super quality binder"
   
    

First-class, A1, magnificent, glorious...
Without this important adjective, the question becomes difficult to answer, but Nintendo themselves consider the SNES to be better than the NES, and who are we to argue with them??? It's basically canon.
SNES > NES