LBRY was commenced a while back -- back when having your own coin was a cool thing to do. LBRY also uses non-standard transactions to purchase and build their URL tree. (LBRY allows you to acquire a named channel to host your content, similar to Youtube.)

In addition, Bitcoin at the time was plagued with performance concerns and high fees. If your vision is to serve and support content to millions per day, you want freedom to address the performance needs of the system.

I do think you could build a similar system on bitcoin; your wallet server could build the necessary indexes. You would need to do some interesting P2SH work to construct the channel hierarchy.

We use the blockchain to store metadata about content that's published to the network. We could not do that on Bitcoin (well, we could do it with OP_RETURN but that's considered bad practice).

In addition, having our own chain means we can tune it to meet our specific use case. What Bitcoin wants and what we want are not the same, so solving both problems the same way is probably not right.