Because it wastes a slot that could be used by another, newer, hopefully more useful topic.
That, and bumps are only done to try and revitalize a topic that died.
The topic died for a reason, don't bring it back. Especially when the topic was a stufffest to begin with.
Bumps in off topic are no big deal, bumps in addons or anything worth looking at will 9 times out of 10 cause stuffstorming.