The BitTorrent protocol uses UDP only when using a congestion control mechanism on top of it (uTP). In fact uTP is aware when the send buffer is filled and when

The BitTorrent protocol uses UDP only when using a congestion control mechanism on top of it (uTP). In fact uTP is aware when the send buffer is filled and when latency is introduced so that it automatically back-off and let interactive traffic some time to be exchanged in priority. The standard BitTorrent protocol uses TCP by default and is arguably worse: since it uses more TCP connexion by nature compared to a client-server file delivery, it have more chances to be prioritized. So BT over UDP with uTP transport protocol is in fact a clever way to balance heavy traffic and interactive one.