Pick A Platform
Well, the mobile space fight between Apple and Google demonstrates why it's so hard for anyone else to get in there: Engadget shows this chart of the extant space of developers:
That overlap - the people developing for the two main platforms - is small. It's likely to stay that way in the short term at least, given Apple's "no cross platform tools" policy. Now imagine you are a third player trying to nose in - Microsoft, HP, whoever - you have to somehow convince people to target a smaller space than the two big players, or offer a way for those people in the middle to add your gadget - all without setting off a reaction from Apple that blows you out of the water (see: Adobe).
Fun Times :)