There are about 50 C5M models flying see screen shot from ADS-B tracking.
The main problem is USAF/DOD/Congress wanted EVERYTHING NEW. Well that takes a while to design and assemble so by the time the first a/c rolled out it was already out dated. The other problem is with so many untried, untested NEW systems the problem multiply exponentially.
One good example on was the Inertial Nav system on the early models. All brand new design. Somewhere north of $100k. Only one gyro. Very fragile. Placed in the nose cone (why nobody knows). Very heavy, ~100 pounds, with all the black boxes. AND voluminous. I think somewhere around 18cuft all told. At the same time Boing was shipping there 747 with 3 (THREE) Honeywell, off the shelf INS systems. Three system that I think ran a total of 20#ea, cost $10kea and only occupied about 4 rack units in a STANDARD 19" rack.