In a lot of countries unlimited download quotas aren't that common, so it's likely a lot of people didn't want to just chew up massive amounts of their monthly quotas just to play a game for a few days.
Personally, i think the progress shouldnt carry over, and never should have even been considered. Seeing as it's a beta and was put there to test, find exploits/bugs so they can patch them out and bring us a better game on release day. There were exploits found and at that point people moaning about their gear carrying over etc should have wised up and not invested so much time in grinding for gear. People exploiting the game they have worked so hard to make fair for everyone is going to push them more towards a reset. Also, as far as i'm aware the XP progression was sped up so people can get a better taste of the game, test out all the abilities and its mechanics etc for feedback purposes seeing as it was condensed into such a short period.
Although, if the exploits weren't there and exploited so heavily once it was found and mentioned, I personally wouldn't have cared if the progress carried over or not. But just for the sake of fairness to everyone I do think its necessary
The main problem with people moaning about the carryover is that this beta was actually being treated as a beta, which makes a wipe likely. Unlike many other franchises have treated their "beta" as a glorified early access to an older game build with carryover. So now people expect this to be the norm.