There are two reasons I see. Trouble in some jurisdictions. But then there is also people who want to secretly use Tor (using Bridges). If for example Chinese (

There are two reasons I see. Trouble in some jurisdictions. But then there is also people who want to secretly use Tor (using Bridges). If for example Chinese (or Iranian or whoever) want to secretly use Tor, which is possible with bridges then it wouldn't be good if they were in a public list. One more reason maybe is that you also want to have Tor running as a weak client, like on your smartphone. On such devices and in such networks running a relay would probably cause issues, both for you, but also for clients. You can't really compare it with Bittorrent, cause what mostly mentions in File Sharing is throughput, whereas Tor also requires low latency for example.