Because tablets are hot right now. So MS thinks that selling a bad tablet that also is a bad ultrabook must sell like hot cakes, because everybody badly wants the "full PC experience" everywhere.
I'm actually pretty sure that with a Core i5 in a 10.6" screen chassis, that this tablet will certainly run pretty hot. 4.5 hours of battery life won't help very much either. It's too much of a compromise. To complete with tablets, you need 8+ hours of battery life. To compete with good ultrabooks, you need that as well. But that adds too much weight with an x86 processor.
"Confound these ancestors.... They've stolen our best ideas!" - Ben Jonson