It's a privacy issue.
Imagine I'm an Evil Doer(tm) and I decide to go to a porn site and download "Sick fetishes 3" just so I can publish the list of IP addresses that were part of my BT swarm.
With BT right now, you as a user are probably savvy enough to know that risk and accept it, but if it was built right into the the browser, a lot of bad things could happen.
It's too bad too, because I'd love to see a much bigger adoption of BT.