One or more of a handful of reasons, in my experience, being part of the Gambit Defense Force since people started hating on him:
1. They aren't familiar with early Gambit or Gambit's solo stories and only know him as the background character and caricature of himself he's been relegated to in the main X-Men books for the past 15-odd years. Current mainline X-Men Gambit does kinda suck, but mostly through inaction, so I can't blame them too much for this. People aren't going to read a solo book about an X-Men character they haven't been given a good reason to read about. Things are getting a little better with this, at least. Gambit is in the top X-Men book at the moment and is being written adequately.
2. Some writers hate on him for being after their time, and/or stealing the spotlight from other characters thanks to the cartoon, and people jump on that bandwagon. Gambit and his fanbase were being openly mocked by writers at conventions for a while, there, like it was the cool new thing to do.
3. This is somewhat related to the last one, but there is a sense that his popularity was unearned and he was overly manufactured to be cool and pushed really hard by Marvel from the start. Historically, though, Gambit was originally created to be a villain but unexpectedly gained popularity before the cartoon, which caused the villain plans to be changed and him to be added to the cartoon roster, so a lot of this is bullshit reasoning.
4. Gambit has a substantial female fanbase that some geeks feel threatened by. This boys club mentality has become less of a thing in recent years, though, thankfully.
5. He has an accent that causes writers to drop stereotypical bilingual phrases into his dialogue, sometimes very obnoxiously. It's fine when Nightcrawler, Colossus, or any other characters have this happen, though.
6. He's kind of subversive as a superhero character, being a thief and generally unrepentant scoundrel, which is naturally polarizing.