Well, some of the authors choose Curse as a place to "actively" update their addons, would be more precise to say actually WoWAce (the addons are only getting syndicated to Curse). Why ? I would reckon, it's mostly a matter of version control system. Usually, if you pick one, you stick with the supporting site, mostly due to technical reasons with the packager support. It's really not anything personal and most of us know each other anyway and don't hold it against each other
I do agree that both databases need some deal of cleanup, BUT, there are instances of addons that are still perfectly working and don't need updates (in all honesty a working addon doesn't need a .toc bump or something, I personally do it, to avoid support questions and save time). Best thing is to report a vastly outdated (and broken) addon and at least here on WoWI, I've personally witnessed that the response/evaluation is generally fast, in such an occasion.