Changing reverse speed would mean every vehicle designed to reverse instead of slug it out to have to be redesigned. A Jackson that can't reverse away from a P4 is going to flat out loose since it has less health and does less DPS.
It wouldn't matter much for the jackson that never bounces shots so doesn't matter which way it's facing. Sure, it makes turretless TDs more vulnerable to flanks but it also makes tanks more vulnerable to a well positioned and supported TD. All too often we see tanks reversing away on a sliver of health while the TD is too slow to follow. Reduced reverse speed would give JP4, SU85 etc. a chance for an extra shot, or a shot at the rear armour instead.