1) They aren’t Tolkien.
When D&D first introduced Gnomes - and I think it was the first RPG to do so, although I may be wrong - all the other playable races were all taken more or less directly from Tolkien. Elves, Dwarves, Halflings (Hobbits without the trademark), Half Orcs and Half Elves making Gnomes the odd one out. The Halflings already covered the diminutive character archetype, while Gnomes themselves had to emerge as an odd-one-out new archetype.
2) People who play Gnomes often play them in a very irritating way. Gnomes are pranksters, silly, squeaky and irritating. While they may be fun to play, for individuals who like these things, it doesn’t mean that others will like to play with them.
I have a long term Gnome alchemist character, based on Paracelsus the real world alchemist, called Alkacelsus. Yes, it is a case in point.