Because they also had two "distinctly different user interfaces in windows 8". The rest as they say is history.
The problem is dissonance with interfaces. Neither microsoft nor third party application makers are ready for step you're suggesting that needs to be taken (completely separate interface for each input method). That would increase workload on application makers who would need to make specifically different interface optimizations and microsoft has shown in windows 8 that their approach is hybridization aiming for lowest common denominator.
Their angle is that they went with too high of a common denominator and went for something that worked with touch but not with mouse/keyboard and now they're going even lower. Which would certainly make 10 an objectively *better* OS for mouse/keyboard combination.
What it would not make it is a *good* one. Because they are still going for lowest common denominator, which is very clearly visible in everything they do down to the latest announcement of "xbox for PC" (yes, yet another games for windows live for n+1st time where n is a large number). Better than trainwreck that was 8, yes. Better than the real competition, which is 7, no.
Which is why they halted non-pro/ultimate editions of 7 again. They know that if they were offering 7 and 10, hybrid lowest common denominator OS would have all the chance of a snowball in hell of surviving the competition with a proper desktop OS which doesn't have to make touch-related concessions on its UI elements. That is the same move they did when 8 tanked hard in effort to boost its success. This resulted in little increase of 8's sales and a massive nosedive of entire PC market. Which started recovering when 7 became widely available after MS reversed the decision under massive pressure from OEMs.
Which is why it's very telling that they making the same move now, as well as yet another "GFWL" move. Smell of desperation is strong once again.