The "Community Promotion Ads" were actually born out of the "Open Source" initiative started on Stack Overflow. So Stack Overflow had the community-driven ads first.
It was the early success and overall good will of the Open Source project that made us think that, perhaps, other communities would appreciate having this feature, too. So the two programs are essentially one and the same, except that the other sites don't have the same sense-of-purpose for those ads as Stack Overflow (I wish they did, actually… but we're not there, yet).
So why doesn't Stack Overflow just include general Community Promotion Ads in the mix?
The Open Source advertising initiative is such a successful program. We have allocated, essentially, a block of "free" advertising to this program. I'd hate to lose that focus by opening that space up to a bunch of other things people might like to advertise. I can appreciate that you want to promote other sites and a plethora of other things here. But there are so many open source project needing our help. I'd hate to lose that focus by diluting that space with other promotions.
