Re: The question of why
We have to keep in mind that Microsoft was terribly rushed here. They needed to come out with an OS that people would buy on tablets, and phones too. They were just so far behind, that they were in serious trouble. No one can rush this though, and Microsoft isn't known for quick moves. How long would it have taken them to re-work Win 7 enough for tablets and phones so as to have it work well all around? Years more time than they had.
They basically went to the ZUNE HD and lifted the more primitive UI from that, along with the programming model, and worked it up. But they still had a problem with convincing people to use it. Win Phone 7+ is evidence of that. While I don't know exactly at what point they decided to do Win 8 the way they did, it's obviously an attempt to get people to use the Modern UI, in the hope it would stimulate both tablet and phone sales.
Being so close to the project, and having the; "Wow! This is a GREAT idea! It will solve all of our problems." syndrome we see all too often, blinded them to the major defects of the idea itself, not only the implementation.
While a few people do love it, most everyone either has a slight dislike, to a strong hatred. So much so that Microsoft seems to be backing off on some of the more important points.