Light just really is not a part of orc culture at all. Never has been in all of their history. And thematically that’s one thing that sets them apart from humans. Humans revere the Light and order and all that, and the orcs are more of chaotic tribal shamanistic types.
For them to adopt the Light would be a very radical change. The tauren sunwalkers did it, but they view themselves as the hands of An’she, one of their gods. The orcs have no gods, only revering their ancestors.
The mag’har having priests is largely due to the Shadowmoon clan who had shadow priests. Not so much for the Light side of things. Especially given how things played out with the draenei, there would likely be no Light orcs whatsoever coming from WoD.
Not that there couldn’t be orc paladins out there, maybe the Argent Crusade has some I’m not sure, but I think it’d be hard to spin a strong enough reason for playability and not be weird.