Browser makers are as performance-obsessed as any game engine - especially Chrome. Did you know, for example, it queues all WebGL calls, posts them to another thread, and runs them in parallel as JavaScript execution resumes in the main thread to start processing the next frame? That's an incredibly complex multi-threaded architecture that helps effectively eliminate the overhead of the graphics driver by overlapping it with the next frame's logic. That is so complex to implement that some native engines don't even try, and we get that for free in Chrome. I don't think you properly appreciate just how sophisticated modern browser technology is, and it's one of the reasons we now see C2 benchmarks in Chrome actually rivalling or even exceeding other native engines on the market. And still, we have people here who think we somehow have a slower or inferior engine in some way due to HTML5. You need to catch up - our HTML5 strategy has worked out great, and that is the reality today; it's not 2012 any more.
Anyways, even just using Unity as an engine is a colossal amount of work. It's basically the same as us writing our own native engine, but using Unity as the framework. It's a forbidding amount of work for a startup and will push back everything everyone else wants by months, and - for what? Performance won't be that much better, if at all (I've read several criticisms of the poor performance of Unity for 2D games since it carries a lot of overhead from being a 3D engine). I think all it gets us is PS4 support. I agree that is important and can see how significant that is for many indie games, but that's not something we can justify that amount of work for right now. There's also a chance they could add HTML5 support anyway, like the Xbox One has, making such a project pointless and extremely risky in terms of investing our resources. And hey, at least you can publish to Xbox One with the Xbox Live Creators Program, it's not like you have no access to consoles at all.