I think the difference is that, in these cases, the cancer itself is what is transmitted. There’s another one (in dogs or cats, I can’t remember) that is sexually transmitted, and is the same way - kills ‘em all once they’ve got it. Human viruses such as HPV get transmitted all the time, but relatively few become cancerous, and even then most are fairly treatable.
I ain’t no microbiologist, though, so take it for what you will.
Thanks.