Lots of technical discussions - but the real answer is that bittorrent/P2P was displaced by Netflix for all but a small number of hard-core users. That, combined with legal threats, and that p2p required volume/scale to work well, meant that the critical mass died. It was a sad day that we, the users of the internet, en-mass exchanged bittorrent for streaming companies.

This, and:

* Asymmetric network links, slow upload especially on cellular

* Traffic package limitations, and both DL and UL are counted

* Some ISP are very against p2p, sometimes it's a government policy (China banned "Residential CDNs")

* NAT