Serious answer: gnomes have a bit of a bad rep as being "joke characters" - especially since Dragonlance introduced tinker gnomes to the wider audiences. And very few players can make joke characters work. Sure you don't have to play a tinker gnome, nor do you need to flanderize it to Krynn levels, but if you don't, they're just magic-using halflings.
Of course, small characters generally are kind of a niche interest, so they wouldn't be popular if they didn't have the reputation for being chaotic stupid, but they get most of the flack.
Gnome paladins have simply never been a strong combo, although in 5e at least they're not a weak combo either. I don't know if they get a lot of play or are just a stereotyped poster-boy for "look how quirky I am!" players.