Social jealousy, that is one of the reasons I think.
Imagine how many reputable and trusted members are currently on the forum and 10% of them are interested in creating a donation thread for themselves for whatever financial reason they face, don't you think that would be annoying.

Then as well as many other users including non-reputable newbie who have financial constraints come and do the same, it's sure to keep the forum filled with private donation thread. Moderators may just want to prevent this kind of thing from becoming a future trend among forum users because at the same time they need to be neutral towards all users.

Anyways the main purpose of the thread wasn't about donations but somehow yahoo posted his bitcoin address there (I didn't follow the progress of the thread) which then made mod ask him to delete via pm.