Because uploading it to a free file hosting service can easily be done via a compromised machine and/or tor (don't know how well uploading large files over tor works though) - maybe even a library/public computer, making it quite hard to trace (though when using a public computer you need to take other precautions, like avoiding surveillance cameras). Also since nothing is made public instantly you have time to erase any traces.
With bittorrent you need at least one seeder that is available long enough to feed the swarm (in decent time), during that time a lot of things can go wrong (if anything goes wrong when uploading to a free hosting service you just start over, nothing lost).
With bittorrent you need at least one seeder that is available long enough to feed the swarm (in decent time), during that time a lot of things can go wrong (if anything goes wrong when uploading to a free hosting service you just start over, nothing lost).