Well, I don’t know the exact reasons. I read somewhere that it’s been a frequent issue. That has either to do with the way the torrent client is programmed and it doesn’t pay attention to the specifics for that case. Or the users frequently get the config wrong.
For example: Since Tor doesn’t support UDP, if your torrent client sends out a UDP packet, it goes over your normal internet connection, immediately revealing your real IP. Whereas if you used a VPN, the packed had been tunneled and that would disguise you.
Also the Socks-Proxy setup is more complicated. Seems to be the case there are just many more possibilities to get it wrong.
I don’t know any reason why you couldn’t theoretically get it watertight. But you have to pay close attention to do it right.
There could be specifics to torrent traffic that expose you in some way. I’m not sure, you’d need to ask a security expert about this. But a torrent download is another kind of data stream than the web-surfing Tor was made for. I know there was research done on Tor. I can only speculate.