I have no idea what mattered to Microsoft, but my view is that MIT is just simpler. There really aren't too many disagreements on what the license requires.
Additionally, if you don't truly plan on enforcing many requirements of the license (e.g., if a distributor fails to "cause any modified files to carry prominent notices stating that You changed the files" (4.b.), will you send them a letter telling them to correct the problem?), then why bother using a license that includes those requirements?
Additionally, if you don't truly plan on enforcing many requirements of the license (e.g., if a distributor fails to "cause any modified files to carry prominent notices stating that You changed the files" (4.b.), will you send them a letter telling them to correct the problem?), then why bother using a license that includes those requirements?