Microsoft indeed tried to get their Tablet to sell over 10 years ago, but their mistake was in adapting Window UI directly into a tablet without adapting the user interface. A pure hands-only tablet has no stylus, no resizeable windows and must support multiple finger touches + gestures.. which is totally different then WIMP. The genius of Steve Jobs is he figured that out early on and directed his engineers to tackle it appropriately- (hence a totally separate UI interface stack for OS X; hence iPhone OS = OS X core + different UI)
Steve Jobs also had the vision to know that people will accept something they've never seen before... as long as the usability is good beyond a reasonable doubt.