Microsoft's problem is that they are late to the party . From all accounts, they have a solid product- but then, so do Apple, Android, and RIM. Why in the world should a potential buyer choose Microsoft over these well established predecessors?
Then there is the major issue of ecosystem. Apple has the best ecosystem , arguably. Android also has a fine ecosystem . RIM is struggling in this department , but it has the enterprise market, for the time being.
What does Microsoft have on offer? Potentially, it has a really great ecosystem,with Zune, Office, and Xbox as starting points, but its in the early stages of weaving it into a cohesive whole. From what I hear, Microsoft has been aggressive in wooing developers, but developers want to go where the market is now, not where it could be.
I foresee a long, slow, painful, and expensive climb for Microsoft in the mobile market before it achieves anything like parity with the aforementioned Big Three.