Existing BitTorrent clients, on the other have something to lose:
* WebTorrent peers are less likely to be connectible (behind NAT etc.)
* WebTorrent peers are more likely to "hit and run" (by navigating away from the page, you stop seeding)
* WebTorrent peers are more likely to favor sequential downloads instead of rarest first (for in-browser playback)
Being incompatible with regular BitTorrent means it does not get any utility from existing swarms (unless regular BitTorrent clients gain widespread support; unlikely).
This means WebTorrent is mostly relevant for content delivery, for use cases a site offloads bandwidth usage onto their uses (like PeerTube, for example), and the site itself always runs a seed.