It's not so much the lack of United knowledge but the petty squabbles people get into when discussing football with rival fans. There are debates on here but they don't usually degenerate into name-calling and petty one upmanship.
Football supporters can be very passionate. When debating with a rival, passion can turn to anger, which then leads to insults and the whole thing just escalates.
Discussing something you care about with other people who also care about it is a pleasurable experience. We can support, encourage, advise and teach each other. Hmmm, sounds like support group...I am reddwarf and I am a Man Unitedoholic!