I can think of a few reasons, but they apply more to the GradCafe's forum more than the database. But these are easy to connect. The most salient reasons:
Suppose you've made your offers public, and have been identified.
1) There is a little bit of a negotiation that can occur (or not occur) when they know their direct competition. Like, if you don't have any other offers, they have no reason to sweeten the pot. They have no reason to offer you something very competitive, because they have no competition. You're showing your cards, which is more than they are doing. We, of course, are willing to share "Hey, this other school is offering me a fellowship for my first year", which is more apropos to contact directly. I'd rather they hear it from me via email than via grad cafe.
2) You may also have made your offers public and ask advice about the program, the pros/cons of the offers, and strategies on making a decision -- or even how to negotiate. Of course these conversations happen and should happen with your letter writers, DGS, and by communicating with their program's liaison. But in a public forum, this can backfire if your comments about a program may be interpreted negatively. It can also look inappropriate, if it is clear your goal is "How can I get one offer to up the ante?" I imagine an AdCom might be annoyed by that.