Lets look at the two side by side, best we can...
This is my opinion, based on training and implementation of the RAT and just looking at the K2.
The K2 comes in double-edged or single edged, so while it may be of similar size to the RAT, in a double-edged design you'll end up self-inflicting during striking with any other grip than a full grip or a full reverse grip.
The K2 also has ridges on the back of the handle and the blade itself, for what purpose? It does not help the blade to be more stable in your hand, that comes from grip design and smooth edges of the grip, which the K2 does not have, when compared to the clearly smoother edges of the RAT. The K2 has a choil, which is the product of either lack of understanding what blades do when they go through clothing/skin or laziness. Any "fighting knife" that has a choil was designed by someone who doesn't understand that skin, clothing and whatever else gets caught (like bone) will get stuck and bunched up there, the K2 actually has two choil's on their double sided models.
Blade grip design is also important, the RAT has a grip that is just slightly thicker than the blade, and allows the spine of the blade (center of the spear point) to be above the grip, there is logic in that based from experience of use (a pedigree, if you will) while the K2 has a thick grip with a huge groove that does not share the same characteristics.
There are other issues that I can see may be a factor but without handling the K2 I cannot speak to (thumb capping, grip being offlined and not allowing for power assisting) and a few others.
That's just off the top of my head.