It really comes down to the subscription-based model being profitable for people that think paying for something means they'll get a better product. People will find an open source solution and rebrand it for a $5 or $10/mo and have it sync to a server somewhere, and then move on to rebranding another solution. You can find a ton of apps in the app store where they do this.
That doesn't explain that they also get a lot of discussion (relative to other posts). To me the story is a bit different. I've talked to a lot of people that take notes daily, and the only consistent theme I've found is that none of them are satisfied with their experience thus far. Until that changes, I'd expect every reasonably good notes application to front-page.
