Because any extension or sufficiently capable malware on your system can change about:config values, meaning that if Mozilla ever makes a mistake in the extension approval process and lets a malicious extension slip through, or you get such malware on your PC, then that extension/malware would be able to flip that config bit and open the flood gates for all malicious extensions. Mozilla would also not be able to change that config bit back in any trivial way, if they notice that they let a malicious extension through.
Only moving that configuration to the compilation step is able to prevent this for sure and most definitely is the sensible choice not just for Granny, but for most tech-savvy users as well, as even only a fraction of those care about unsigned extensions.
Months of public discussions have been held on if this can be done in a more (power-)user-friendly way without comprising security for the vast majority of users, who really do not care for unsigned extensions either.
Yes, these discussions would have concluded differently, if those 0.1% users who actually care about unsigned extensions made up 80% of Firefox users, but acting like this decision with a motivation that goes well beyond just wanting to not do as much maintenance is somehow indicative of how much Mozilla cares for its power users, is just ridiculous.
And just look at the malware fest that is the Chrome Store. This is what Mozilla was able to prevent by requiring extensions to be signed.
Lastly, specifically because they now have a functioning way of controlling malware, they can actually be a bit more liberal in what they allow to be configured via about:config and there is now no real use case anymore where it would make sense to move an about:config value to a compile flag.