"without doing much of the kind of work I actually enjoy."
Under him The Magazine as a product probably would stagnate like Instapaper did. Then he would have sold it anyway.
And, advisory role usually means a decent chunk of shares too. With what Glenn have in mind, this might be a smart financial step.
Given the success of instapaper and the fact he'll have done fine out of Tumblr, I suspect Marco has no need for that and would rather devote his time and attention (something can be distracting even while taking little actual time) to something he'd be happier doing.
The details of the sale aren't public either, and he's now an "adviser" to The Magazine. He may still have some equity in the company. One could also analyze the profits from a sale compared to estimated profits down the line, and feel that they would rather cash out now, to pursue other interests. A lot of "savvy" business-people and businesses divest themselves from businesses that they feel have outgrown them or are not a part of their core business model.