In order to kill two birds with one stone. The other thing is that the loss of Halo wasn't Thel's fault solely. Upon finding Halo, the Prophet of Stewardship attempted to invoke his religious authority, claiming that Halo is a military matter, not a religious one. Thel 'Vadamee argued back, claiming that because humanity was still present on the ring, it is a military matter. This resulted in a division of command, with the Prophet of Stewardship ordering around Thel's troops, directing them away from important locations, and leaving the Covenant military presence to be a mess.