I have never known nor heard of a Mission President who was in humble circumstances. All were in fact wealthy by anyone's standards.

Why is that?

Can financially withstand a multi-year hiatus. Success in business is also very similar to success in missionary work as Paul illustrated with his "be all things to all people"; it's all salesmanship as God entices and does not force.

So is being financially wealthy more important than being the best man for the job?
Or do you think the Church has enough wealthy people from which to draw Mission Presidents?
Or are people with humble means just not Mission President material?

The second. I'm not saying a person of humble means is never called or can't do it, but it's not likely such a one has managed many people.

My father was a mission prez, and he was also a man of modest means. But it is also true that he worked for BYU his whole life (he was a professor), and he was working there at the time of his call, and when he returned from being mission prez he went back to work there.

(By the way.... I am active LDS, temple rec, I did my home teaching last month, AND I AM A PROUD DEMOCRAT.)

My husband's mission president and wife (who attended our wedding...neat couple ) were not particularly wealthy. They were both retired school teachers, so they had some savings, and were able to handle the unique conditions of being away from their home for an extended period.

The calling is so unique that it just requires the right types of circumstances.

I thought Mission Presidents were among those that received "modest stipends". At a minimum they live in the mission home and drive mission cars

It seems one of those circumstances is wealth.
It feels kind of wrong that ones financial circumstances are a determining factor in a call from God.

Or come to the attention of the General Authorities. Socioeconomic status comes into play for Church positions from bishop on up, with a notable exception for members of the Church Educational System. The location of the Mission is a consideration as well. Much like our country's system of ambassadors, the Court of St. James is a plum position. Expect to see generous donors there.