Different markets. ABB is not in the networked control business in the same way A-B is. Sure they have it available, but no market share for controllers, so they have to make theirs adaptable to other control systems, which isn't as easy to implement.
The 755 is INTENDED to be used as a network connected drive connected to A-B controllers. So your desire for an analog output is in conflict with the person using it as a network connected drive and saying "Why do I have to pay for all this I/O that I never use?" If you want I/O on the drive, you select the 753.