Beyond expertise, HTC doesn't offer anything to Microsoft worth buying. The brand isn't worth buying. The factories and assembly lines aren't worth buying. HTC have no research projects worth acquiring, nor do they own a single software franchise or service of economic importance. We might argue price, as HTC would probably cost Microsoft less than a third of what it would cost to buy Nokia, but price is never a good reason to buy a company if it doesn't complement your current operations in meaningful ways.
If Microsoft buys a manufacturer, it will be Nokia, which has something to offer Microsoft in all of the areas I just listed.
The only way a HTC purchase by Microsoft would make sense, is if Nokia terminated their WP efforts.