Why would only a specific model of drone be allowed to fly over crowds?
CNN has applied for and received multiple waivers for 14 CFR § 107.39 Operation over human beings as allowed by § 107.205 List of regulations subject to waiver.


Issued August 29, 2016 &mdash; This allows "Small unmanned aircraft system (sUAS) operations over human beings in accordance with the CNN UAS Operations Manual, Fotokite Pro 1 Operator Guide, and Fotokite Pro 1 Specifications Sheet." It allows operation "over private or controlled-access property with permission from a person with legal authority to grant access" and prohibits operations over open-air assemblies of people.
Issued July 20, 2017 &mdash; This allows operations "conducted over human beings for the purpose of closed-set motion picture and television filming". Make and model of sUAS not specified, but all operations must comply with the operator's manual.
Issued October 13, 2017 &mdash; This waiver is granted for the Vantage Robotics Snap only, and allows "operations over all human beings (including Nonparticipants)". It also limits the altitude when operated over people to 150 feet AGL, as opposed to the normal limit of 400 feet AGL. I assume this is the waiver you are interested in.


The simple reason why only one particular model of UAS is allowed is that CNN only submitted that model for waiver and no others, and the FAA approved that.

This is a somewhat unsatisfying answer, but neither CNN's nor the FAA's reasoning is included in the text of the waiver itself. The rest of this answer is speculation on my part.

As can be seen on Vantage's specifications page for the Snap, the Snap is reasonably lightweight (1.37 lbs / 620 g including battery), has caged props, and has a high-quality camera. (The camera isn't a matter of safety, but it is presumably something a cable news provider is interested in.)

If a prop were to fail catastrophically, the cage would hopefully contain at least some of it, thus eliminating risk of a crowd being hit by sharp objects. The height restriction of 150 feet AGL reduces the potential for injury of a nonparticipant should the sUAS fall on them, and the prop cages prevent contact with moving parts.

I suspect that CNN applied for a waiver specifically for this model and the FAA approved it based on the following:


It's specific. The more general the application, the more likely it is to be rejected. If you know what you're working with, you can evaluate all the parameters and come up with a safe solution. If there are more variables, it's harder to reason about.
It has certain safety features as discussed above, which make it more suitable for flight over nonparticipants than some other models.
It's standard. CNN has a single platform to train their over-crowd operators on. Once again, this also has safety benefits, because the operators are extremely familiar with one model of unit, instead of broadly familiar with many different ones.
This particular model of sUAS had all the features that CNN wanted and that the FAA was interested in for this application. This may include such factors as weight, safety features, well-written operation and maintenance manual, and ease of maintenance. Additionally CNN may have liked the cost (the FAA wouldn't care about that).


The FAA maintains a list of all Part 107 waivers granted.