I’ll just point you over to the post on ITM about this, where agitation against the venue originated (and where I’ve posted a response to the active discussion there). I posted on this earlier in support of the petition to move it. I’m disappointed that the EC’s statement does not actually give a reason for the vote; it merely says that they talked about a range of issues connected to this:
In reaching its decision the Executive Committee discussed at great length a wide range of issues, including its fiduciary responsibility for the Academy’s endowment, the appropriateness of making collective political statements, the precedents that would be set if the Academy canceled the meeting, the scholarly effects of canceling the annual meeting, the work done by the Arizona programming committee, the difficulty of finding any alternative meeting place, the timing of cancellation, and the possibility of legal challenge to Arizona’s legislation (which in fact occurred on 28 July).
After weighing all these issues, the Executive Committee has voted to hold the meeting, as planned, in Tempe.
I’d suspect that this is because it was a divided vote, but that’d be just speculation.