Last year it was reported he was earning $600,000 from the job, as am MP he's be on about 10% of that. You'd need an extremely high level of principles to give that up, and he's not got that.

Here's a bit more on it

http://www.dailymail.co.uk/news/article-3740145/David-Miliband-urged-UK-politics-standing-tragic-Jo-Cox-s-seat-Jeremy-Corbyn-Labour-leadership-race.html