As people have mentioned before, orc priests come mainly from the idea of the Shadowmoon clan, who has a past with the use of the void, and in general most of the Mag’har priests would be void practicioneers.
But I wouldn’t be so adamant about they don’t use the light. After the events of WoD there was a cultural exchange between the orcs and draenei, Geyarah herself saying she was taugth (to read I think?) by a draenei.
So it wouldn’t be so strange that some orcs embraced the light willingly. Most of these orcs now would be part of the Lightbound, but there could be some who stayed loyal to his race and still use the light to fight, even is they are shunned by their colleagues as the void practicioners are shunned in human society.