The user has declined.
Apart from being a great mathematician, he cares a lot for the community.
As can be seen for example from his good contributions to the list of questions to the candidates herehere, herehere, and herehere, he nicely represents and defends the original traditions and standards of the community, which made Math SE a good place with a nice academic atmosphere for people who want to help each other learning and doing mathematics together.
BMS: I’ll follow Martin’s lead by commenting here. I appreciate the nomination (to say nothing of the undeserved compliment on my mathematical skill!), but I’ve no desire to be a moderator. I’ve successfully held a number of positions elsewhere that required similar skills and willingness to subordinate one’s own views to a large extent to prevailing community standards, and I think that I could do a decent job, but that same experience has taught me that I wouldn’t enjoy it. I’ll stick to answering questions: that’s both my forte and something that I greatly enjoy.
There are other reasons for me to decline as well. It’s no secret that I’ve a view of what MSE should be that is not universally shared. (We all do, of course, but mine is further from the middle ground than most.) I fear that this has made me something of a polarizing figure, and as such a poor choice on general principles. Moreover, while I dislike getting involved in the pie fights, I want to be able to speak up for that view at will; as a moderator I’d not feel free to do so.
Finally, I had to walk away from MSE a year ago, and I can’t promise that I won’t need to walk away again at some point.