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Digital instruments are easy to program and don't take much computing
resources. Converting the display to a form fit for human consumption
take more computing and programming horsepower.

Exactly right. Plus two additional problems:
1) Most modern general purpose computers have voluminous operating
systems and take too much time to cold start (or boot up), even if ROM is
substituted for the disk drive. That means a lot more programming.
2) Presently, there is too little standardization, especially of the NAV
equipment. And integration of the NAV display(s) is a major reason for
considering electronic displays.

So it's not that we necessarily prefer mechanical instruments, but we
certainly have reason to demand that any replacement be at least as good in
all ways important to a pilot, such as:
1) Ease of comprehension.
2) Similarity of controls and displays in aircraft a pilot might fly.
3) Redundancy--at least as good as our old electrical plus vacuum.
4) Immunity from "wash out" in direct sunlight.