Someone would put up say ronaldo and everyone would send up 4 bronze players. You would have to go through tons of useless offers.
On the old forum people with thousands of posts and who you thought were trustworthy would trade players say ronaldo for a bronze to try it out and then keep him and never return to the forum.
Also jp flathead who ran the ultimate database tracked them and said it was something like .05 percent were successful so very few used it for the hassle.
So it would be easier to just make them tradeable to avoid all the hassle but then why have them be untradeable.