In my admittedly amateur opinion, part of the problem is how many people preconceive of the 'Tueller drill' scenario. Why is it that we all assume that the attacker will be approaching from in front of us? What if he is charging in from the side or even from behind? if he's coming from the front, why wouldn't he stroll for the first few strides and then draw his weapon and charge in for the last two or three strides? Is it realistic that most of us could recognize the threat under these parameters and draw then fire before the attacker shoved a knife in us?
In a Kali class (Filipino stick/sword fighting) I've been taking off and on for the last couple years our instructor had us run the Tueller drill one evening after the regular class had let out. At first, we ran it as everyone probably thinks of it; an attacker charging from a standstill head-on to the defender. Knowing the drill and having the advantage of looking right at our attacker most of us were easily able to mount some kind of defense, even though some of the younger more fit guys could easily cover the 20 ft. in far less than 2 seconds.
Then our instructor changed the drill, putting the attacker behind the defender. Another twist was to have three guys in a half circle around the defender yelling at him then one charging in at random. This type of thing greatly increased reaction time, making an adequate defense much more difficult.
It turned out that our instructor had seen this video with Doug Marchaida and instructor Zero on Youtube and decided to see how those of us that were familiar with the TD reacted to the drill with a few twists.
https://www.youtube.com/watch?v=2fjMpn7JCJ0
If you don't know who these guys are, Doug Marchaida is an edged weapons specialist with roots in arnis and kali sword fighting, as well as probably a half-dozen empty-hand martial arts. If he was the one attacking you with a knife, it would be a nightmare right out of Hollywood. Instructor Zero is an Italian Spec Ops combat instructor. Not much else known about him other than his videos and that he holds the highest security clearance levels possible in Italy. And he can really shoot really fast from any position.
In other words, most of us aren't going to do a lot of the things you'll see in this video, but it does effectively show us just how difficult the attack from 21 ft. can actually be.