Yeah, this is definitely what I saw with Sparrow. For it to become popular it has to be adopted by a substantial number of people. However, I find that only really tech savvy people cared about Sparrow and its streamlined interface. When I showed it to other people, they were reluctant to use anything else because they were afraid to move from their Apple Mail. I'm guessing if Sparrow had existed for windows it would've been the same thing with people using Outlook.

And then there's people who don't even use e-mail, who think it's outdated and prefer PM messaging as a form of communication. It's easy to see why a lot of these promising mail client apps don't really take off the way other apps and services do.

Nevertheless, Sparrow will always have a home in my Mac, as long as it keeps working on OS X.