That's probably because of Sentinel's origins in FFXIV as an actual class and not an ability (yes I know it's a traditional ability that appeared in past FF games, but in FFXIV 1.0 at launch it was actually a special sub-class you got levels in if you equipped a shield for those classes who could). Hence because GLA/PLD use shields and MRD/WAR doesn't: no Sentinel for you. Rampart probably has the same line of thinking (I'm just pulling things out of my head here, so that might not actually be the reason why the development team did this, but it does seem to ring true.)