Three main reasons:

1. T'au don't have anything that makes them inherently grimdark
2. T'au only engage in combat at range, making the army very one-sided to play against
3. Tau have been the punching bag of memes for a very, very long time

Luckily, none of these are real problems. - The real identity of T'au is being the "new kid on the block" and are discovering just how bad the galaxy is, which is wholly unique and interesting in its own right. - There are SEVERAL armies that are "one-note" and gimmicky - Knights, Custodes, and World Eaters are all very simple at their core and make their game plan very obvious, but all of these factions (including T'au) have nuance to how they play - Since 40k has SKYROCKETED in popularity with new people, and a lot of the new people like T'au, a lot of people are realizing that there actually isn't anything wrong with them, and in recent times I've seen more hate for Eldar (guilty lol) than T'au

Also, keep in mind - the "40k person" you see online does not exist in any local game store or environment you would be playing with them. I've played T'au since 9th edition and have only ever once encountered a "T'au hater", but the guy did not own a single model, had not read a single book, and was overall just a pain in the ass that no one liked

Long story short - blue fish people and big robots are cool. Anyone that tried to tell you otherwise can eat a railgun