Because they both have an existing desktop product, both of which are dominant in their own sphere (Microsoft: desktop computers, Canonical: Linux desktop compu

Because they both have an existing desktop product, both of which are dominant in their own sphere (Microsoft: desktop computers, Canonical: Linux desktop computers), and they'd really like to leverage that advantage to get into the touch-based market, which is predicted to grow massively while desktop is stagnating. I guess there are also a few products where both input paradigms make sense, ie. the tablet/notebook hybrids, you wouldn't want to entirely different user experiences on a single device depending on the currently chosen input, so you make a hybrid user experience fitting the hybrid device. The other choice would be simply not supporting hybrid devices, but clearly that is not what Microsoft has chosen to do.