Because the IOTV bodyarmor with MOLLE does not give you many options for a large knife like the K-Bar.
Here I am wearing the Ek Model 4 Commando Knife with the blade portion riding behind the magazine pouches. Even with the blade tucked up behind the magazine pouches, the knife is too high on the chest. It makes for a very slow and cumbersome draw. I can't go any lower with it and I don't have the clearance to wear the blade upside down as the magazines are in the way.
The easy solution would be to carry a shorter blade that's faster to draw, and easier to carry.
Other solutions would include: wearing it inverted on your back, angled on your front - handle point up and forward, horizonatally under your mag pouches, etc.
I know a LOT of guys who carry large blades on their armor. Almost none wear droplegs, and none run a blade there.
That blade might look fast while you're standing around with your hands at your sides at the chow hall...but in a fight where your hands are on someone, or are struggling with your carbine in front of your chest...that drop leg will seem a lot further away.
Additionally, you're restricted to one handed access. You're not getting an item off a drop leg on the other side of your body. In the 1 picture, it looks like you're a right handed shooter, and your blade (only other visable weapon in the pic) is setup for a RH (only) draw....why?
Lastly, the location and position of your handle will change as your leg position shifts...again, what might seem fast from a stationary relaxed position can be very different from a more dynamic deployment.