Because for the most part, threads do not discuss “halacha” in its black and white form. What we pick apart is the interpretation of halacha, or minhag (which we mistake for halacha).
As you may or may not know BP has an eruv war (Not going to say which side I’m on), with each side having serious moreh horoahs backing the pro and con team. Who is right? Depends on how you interpret the halacha. Team Pro will “prove” that the eruv is A-OK; Team Con will “prove” their position with just as much conviction. And believe me when I tell you, both sides have Rabbonim with real credentials.
So.. who is right? That’s a personal choice of which Rov you follow. And once we start to get personal, the fur starts flying!
I picked the eruv, but the same applies to every “hot button” topic, like the tznius thread that was running strong earlier today