Plain and simple -They fear litigation.
Any acknowledgement by Delta or DALPA that Midwest is a part of the current merger or somehow their "responsibility" would trigger several laws, one of which is the new employee seniority rights law passed in response to the TWA/AA debacle.
The Delta MEC can't negotiate integrating the Midwest pilots. Let's say they offered a staple to the new seniority list? Many Midwest pilots would probably jump at that but it would only take one pilot to sue and claim his Allegheny-Mohawk rights were violated.

You can thank the AA/TWA flight attendants for the Delta MEC's paralysis.
Along with Kit Bond and Claire McCaskill. They meant well in trying to stop any repeats of AA unions bullying the TWA people but the effect was also to prevent one union from throwing a lifeline to another without opening a giant can of legal worms.