Its the same fascination of X3 or 300 or 100. Microsoft is a technology company, not a marketing or financials company (cough...Sage...cough) and they feel the AX technology is the most cutting edge of the 4 solutions (not that it makes it a better product for their customers). it also means microsoft can go in and act like a real player against Oracle and SAP. Microsoft has always wanted to push into Enterprise apps and there is no love lost between Ellison and Balmer, so this has helped fuel the AX push.
Plus realistically, if they are ever going to narrow down these products into one, it's pretty obvious AX is the most futuristic platform. The problem is, after all these years, its still way too complex for the small and lower mid-market segment and is still missing alot of functionality compared with something like Epicor without relying on alot of 3rd parties