My impression is that the suspension is less about this specific incident/this specific fraternity and more to do with problems with the general fraternity culture there. According to the Student Affairs VP, over the past couple weeks alone,
the university has gotten at least 15 complaints over hazing, sexual assault, and other problems--and at least 6 of those complaints are serious enough that they're being investigated by the police. To me, that does suggest a crisis that's dire enough to take drastic measures.
The NPHC and NPC groups are either not engaging in the same kinds of practices or nobody is bothering to complain about them doing so, so it's not surprising that they wouldn't be subject to the ban.