You'd have to ask that guy, I suppose. He posted it to two different communities, and neither community's modlog show that post being deleted by a mod, so they definitely removed it themself.
It was me i was the guy and i deleted on and the other one a mod banned because i edit the poat to something like deleted forever loser before deleting it . Like sheesh the mod banned it in 3 seconds or something good for them and i appreciate the dedication. Anyway why is me deleting a post such a big thing some user came to ask me in another post i made about it and i told them to dm me but they never did.
Because it's bad etiquette. You're removing the replies of everybody who bothered to reply to you, as well as denying anybody else the chance to read the conversation. Especially if it's a question that might be of interest to other people.