The reason SoF is concentration and Mage Armour is not is because SoF stacks with anything and everything - it's a flat bonus. Mage armour is not. It dispels if you wear armour, and it doesn't stack with other class features that give you alternate methods of calculating your AC. A monk with 16 Dex and 16 Wis would gain no benefit at all from mage armour... but they'd get +2 ac from Shield of faith. The application of concentration and non concentration effects is better balanced that you think. You just have to really examine the why of it sometimes.
For those curious, I'm hoping to make a more in depth post about it soon, but the reason that slipping prone auto-breaks you concentration with no save is because 'Prone' in Larian world also inflicts 'Unconscious' as part of itself... which in turn inflicts 'Incapacitated' as part of itself. Yes, it's completely ridiculous.
