Sailingfun had a good post on this airplane order. He basically stated the 737-900ER fills in for some 757's as they retire.

Delta wants a 140 seat airplane, which is larger than the E190 and the C Series.

Now my opinion:

Delta apparently wants to replace the MD88 with an airplane of similar capacity. That leaves an enormous capacity gap between 76 and 142/160 seats. IMHO the company is waiting out a worse downturn which might give them the opportunity to close the 100 seat order during concessionary negotiations. The Company is biding their time until attrition picks up and the scope loss would not be felt by those currently on our list.