Hi,
1. Because of the general belief by Phil (and others) that DBM allowed too much maneuver.
2. Because in DBM it was often used as a way to circumvent PIP penalties.
Note that in DBMM Generals, Regular or Irregular, CinC, Sub or ally, are exempt from the Irregular ineptness clause. I can't remember if that was the case in DBM.
Hope this helps,
G^is,
JohnG