I'd wager its because most videos that we watch ends quicker than the confirmation time on the Bitcoin network.
Think about it............Say you're watching a video for 1lbc (25 cents or whatever) for 2 minutes and you close it and watch something else, another 1 lbc. That is a ton of micro-transactions that is pretty much screwed if you're using bitcoin. Hell, I don't even think Litecoin can handle that amount of transaction being kicked around a system.
Since Lbry wants to be its own little ecosystem, I don't mind that it has its own token (LBC is not an actual coin like, if you wanna mine an actual profittable coin, go for mona or something).
What I do have issue with is the 400million premined lbc: for a platform that touts decentralization, we're putting an awful lot of faith into the hands of very few (acne covered) developers