They need developers and they need to make people feel ok with the new interface before buying a tablet. So it made sense to put the same UI in windows 8 so people can try it and then feel comfortable and buy a related tablet.

Problem is: the most of the people hate metro ui, the medium user can get the difference between w8 and wRT and really a incredible small part of the total windows developers joined the ship of winRT/JS and no one of the "main" contributors