Its easier with software
Because of their past failures, and because MS has to work with a HW company, its easier to pull off with a software only product.
I'm not suggesting they abandoned tablet computing, just that they knew they didn't have a hope of a timely entrance into the market, but they decided to muddy the waters which probably hurt iPad sales, at least for a time. Meanwhile they will retreat and regroup.
Apple's advantage here, of being able to build the HW and the software in house and not rely on another company does pay off in time to market.
MS's advantage is that since virtually every enterprise uses MS software somewhere in their organization, they have some captive customers who must consider anything that MS creates, in order to reduce the complexity/number of vendors.
