In a lot of ways, that question is just about as meaningful as "why are there electrons?" Looked at from the right angle, you can say that there is only one speed at which everything always moves through space-time; the only difference, really, among speeds is how much of that speed extends timewards from the moving thing's perspective. (What space-time itself gets up to, and what it is apart from something we experience, is a different set of questions altogether.) The idea that we ought to be able to understand everything in principle is a horse long out of the barn.
Its definitely an interesting question. Something that limits all, moves uniformly no matter the point of view, doesn't interact with itself but does with everything else - its a singular feature of physics.
If I understand you right, one can wonder things like, maybe 'light' is really 'time' in some sense, not moving uniformly but instead the actual clock that drives the universe. Stuff like that is, at the least, interesting fodder for bull sessions.
