No one took the exam because it was effectively impossible.
To become a PE, first the candidate has to pass one of the Fundamentals of Engineering exam to become an engineer-in-training. Except, whoops, there wasn't ever a software specific FE exam; the most relevant one is the EE/Comp. E. exam. Take a look at the list of topics: https://ncees.org/wp-content/uploads/FE-Ele-CBT-specs.pdf Most developers aren't going to pass that even with a CS degree.
Secondly, you need 4-8 years of supervision by a licensed engineer. Again, whoops, there are barely any software developers with a PE license, so who would they get to supervise them?
Only then do you get to take the PE exam for software engineering. Frankly, the situation was so absurd that one has to suspect that NSPE didn't want to certify software developers as PEs.
It was discontinued because less than 100 people applied for it over the course of several years.