People generally don't hate Raichu. Simple as that.
I don't know who you were battling, but from my own experience, people don't hate Raichu. Your guys were probably just exceptions.
What does happen is people simply like Pikachu more. Pikachu is the mascot of the Pokemon Franchise, has been merchandised many times more than every other Pokemon and so on - it's only natural it would be one of the, if not the most popular Pokemon. Not to mention it's the main character along with Ash in the animes. It's also generally considered 'more cute'. Not saying Raichu is bad tho, it's pretty cute too, just general opinion is Pikachu is cuter.
>should be banned and then always say Pikachu is way better
Referring to this, being "banned" has several possible meanings. The most likely one in a competitive scene refers to it being banned from the tier, so basically it gets bumped up another tier. If that's the case, there's a contradiction already there - they say Pikachu is better, but Raichu should be in a higher tier, which is logically for Pokemon which are better?
Just ignore those people.