awesome, but there's lots of reasons.
1) Portability. Forget about mobile devices. Forget about easy distribution. Gosu apps are supposed to be compatible with Windows, Linux and Macs, but they never really are.
2) It doesn't provide very much. It's great for handling input and graphics, but all the mechanics and physics and stuff is up to you. I actually like it, because obviously it makes you learn a lot in the process, but there's a lot of people out there that just want to create something without having to worry about complicated logic and algorithms. Compared to other game libraries/frameworks Gosu can't help you much with that, and I'm not even talking about monstrosities like Unity, but eg. Phaser
provides you with not just one but three! ready-to-use physics models.
Sure you can use external tools like Chipmunk etc, but they are not neccessarily always well documented, and tend to break compatibility even more.
3) Handling sound is oversimplified. It's enough in most cases, but if you want something more complex like focusing your actual gameplay around sounds or implementing dynamic music, you might not be able to (or at least it would be tricky).
4) Debugging. Being an enterprise developer, I learnt to appreciate the importance of a good debugger. OK, you can debug Ruby scripts. But you don't want to. And if you want an IDE that makes this process at least half-sensible, you have to pay (quite a lot).
5) An action/reaction loop of "it's not very popular because it's not very active because it's not very popular because...". You know what I mean. It's being developed by a single fine gentleman, who obviously doesn't do it for a living, so it's quite a slow process. There's something going on all the time, but it's not much, and there's so many features I'd love to see in Gosu, and they're not quite there yet and... Blah blah.
Also, the community isn't very active. I mean, there's no problem finding help if you're stuck making something (although sometimes the answer is "you cant"), but you'll notice there's not much going on on the forums etc.
Ask me why you should
use it and the list will be twice as long, but what I listed above are quite major no-noes that keep a lot of people away.