I think the reason why getting the wrist may be so popular is simply because it is close enough to the weapon to allow one to control the arm effectively as well as execute a possible technique in the event all goes well.
If it does not go well then you at least have a hand on the weapon arm and this acts as a guide to know where the weapon is in relation to your body and helps decide your next option for technique.
Going for the thumb however, especially how we use a knife with the blade upwards if stabbing / thrusting (a method also taught at a DT knife course I've been to) is a dangerous proposition if the knife fighter is moderately quick. You will probably lose some fingers if not have a serious gash in the hand or wrist when he tries to recoil the weapon after he detects you trying to take it away.
Getting the thumb also requires a good deal of accurate fine motor skills imo. I instruct students to never try to "catch" a weapon hand (using the fine motor skills in the fingers) but to use tegatana (blade edge of hand) as a guide that leads to a quick and strong following grasp. It's a 2-stage movement that one can easily master with practice. Those who try to catch the hand, which may be necessary in the event of the thumb due to its protected position on the inside of the handle, often end up with either a slit palm or wrist when the blade is quickly recoiled. The idea may work in cooperative kata practice but has failed so far against a tsuki under even medium resistance in our training methods.
The only workable option I see is if one is able to control the wrist or forearm first and then go for the thumb after having limited the mobility of the forearm, wrist or hand, which is usually the fastest moving thing in a knife attack and difficult to judge without some other control over its movement.
Just my thoughts.
Of course I can be wrong.