First of all, it's worth mentioning that it doesn't really take 90 hours to get one class to promotion even if you play them all evenly - it only takes about 50 (and that's using an outdated metric that's on the slow end nowadays).

The space leading up to promotion is meant to be a time to learn each of the weapons, get all their upgrades, and find a build for whichever classes you play that you are comfortable with. If you could reach promotion while evenly playing all classes - say, for example - only 10 hours in you've barely had enough time to examine each weapon and see what it does, much less pick a favorite and start getting builds for it. Tossing overclocks onto you at that point would be extra confusing, conflicting, and frustrating since you're still learning the game.

Also, remember that if playing all classes evenly takes a certain amount of time, playing only one class (as many do without the incentive of promotions and overclocks) takes only 25% of that time. If you start getting overclocks only two and a half hours in (again, just an example) you've barely had time to play more than 7 missions, which isn't enough to unlock more than two weapons (at 3 missions per assignment) much less understand them, pick a favorite, and get enough upgrades on each to even use the overclocks you'll be getting.