The Value Proposition.
Regardless of what SoC architecture they chose to use, Microsoft needed to tailor the tablet value proposition to their fan base; the desktop user. Al least, initially.
Development of Windows RT catered too much to a saturated, and well served consumer market. Sure, it gave a curt nod to productivity (perhaps too little to not be an affront to any hard core Windows user), but, if a desktop user needed to learn a new UI anyway, might as well go with an incumbent.
Microsoft saw a big market, and wanted to go there, away from what they knew, away from a base that already appreciated what they were all about.
It’s kind of like a girl scout that has a limited time to sell cookies. She could go across town, to the high rent district, and compete with the resident girl scouts there. Or, stay local and visit the homes of families that watched her grow up. It may not be the neighborhood of the mega rich, but should allow her to reap all the benefits of their affection.