From what little I know about how Tor works (I might be wrong about some things), it seems like if more users were incentivized to use Tor for downloading and sharing files, the number of endpoints would increase, making it less likely that your traffic would end up in an evil logging/scanning/sniffing endpoint, and the increased amount of traffic would make it harder for malicious third party to sift through so much data to find someone's specific data packets.
I've seen installers which simplify setting up VPN and whatnot that would benefit user, but which would deter new users if they had to configure everything manually. So it can be done, and regular users may only need to hit "install > yes > yes > ok" and everyone on the Tor network would benefit tremendously. It seems they only need to say "we're moving to Tor!" and it's as good as done.
So why isn't this a thing in 2017? Are there some hidden downsides to inviting millions of new users to join Tor? And are there some problems associated specifically with bittorrent filesharing traffic over Tor? And if Bittorrent is not a desirable way to share data over Tor, what would be?