There's been a lot of talk about this, but I feel like no one has really mentioned the big reason. Bungie needs control over their game. If they let you hang onto things, you're getting in the way. If they wipe everything clean, they can tweak and even remove things without worrying about the effects it may have on current players. One example of this would be a change in weapon rarity. If it had a 25% chance to drop in the beta, but they change it to .5%, their rare weapon is not nearly as rare as they're attempting to make it. That's just one example, but you should understand what I'm talking about without any at all.
But how will they know what needs to be changed. I can understand for gear upto level8. But after that they can't tell cause we can't use it. I have a legendary hand cannon that I can't use cause it's level 20. If we can't use it how can we beta test it. If we can't test it how can they determine if it needs to be changed.