i think its probably because the admins dont want a swarm of greedy people donating all they can just cause they think they are getting something good out of it. -remembers the jolly slippers crisis of 2003-
I personally think they didn't annouce it because they want to see how many people would donate out the goodness of their heart rather than thinking they'll get something in return. Usually if there is an annoucement about it, it means you will get something in return (like at Christmas 03, if you donated to the christmas tree you got something in return). To be honest I think the people that donated a certain amount and over will get something (for the fact Gambino says he won't forget this), and if it's true, I hope they annouce it after the mansion is complete this way people are not going to run over to Isle of Gambino to donate just to get something in return.