Well Angel Beats was something they worked with together on with Key so of course Mari was out and Key's writer in Maeda Jun was in. Maeda is Key so you had to have in that position... As for why it sold and the rest of their works didn't? It clearly appeals to the current trends a lot more than their other productions.
Maybe Iroha would've have been 2 cours long without Angel beats.. if doing an Angel Beats every now and than gets us some 2 cour long original animations than I'm all for it.
Also looks like Iroha's already been animated upto the 10th episode.
http://animapple.blogspot.com/2011/0...completed.html