Bill Gates (and with him, Microsoft) has been nigh obsessed with tablets since at least the mid-1990s. I think the trouble is more that MS entered the tablet market way too early, and got bogged down by hardware limitations of the era: the CPU etc. had to be something from the laptop world rather than from the PDA world because the latter was not capable of driving a big color display. Laptop CPUs were expensive and relatively power hungry. Big color displays were expensive too, so because of the cost people expected a tablet to replace a laptop. This probably led in part to the decision to run a desktop OS on tablets rather than come up with a new UI model and start a new app ecosystem. Rather than going with the (expensive, crappy, or both) touchscreen technology of the time they mostly used Wacom digitizers. The tablet that so resulted was doomed to be basically a niche product.

I think you could say that MS is in the same situation with respect to the modern tablet market, that Palm was in with respect to the smartphone market about four or five years ago.

As for the other phone makers, remember Nokia and others have been trying and failing to do the tablet thing for years too. Remember all the clamoring (for years) for Apple to introduce a tablet? It was not because all the Apple rumor bloggers had some clairvoyance about the market that Steve Jobs did not - it was because seemingly everyone else had a tablet out, or would soon, and people were afraid Apple would get left behind.