You don't put them in a turbo because a turbo intrinsically has more torque - it's a characteristic of a turbo. Also, a turbo makes its max spool at lower rpm than a CF blower - well, when the turbo is sized properly.
For instance, my CF blower makes ~9.5 psi at 6500rpm, but only like 2 or 3 psi @ 3800 rpm. So, they probably have a transmission that changes the effective "pulley size" or internal ratio of the gears while the input wheel is spinning < 4000 rpm or so, then once it reaches 4000 rpm it switches to the 1:1 input wheel and runs the Rotrex on the default ratio which I think is 10:1 or so. My asa blower is 15:1 so when you turn the input pulled one rotation the impeller spins 15 times. This means that theoretical max boost is higher, but you have to be careful how fast you spin the impeller. My unit runs a 150mm input pulley which is MUCH larger than the ~2.5" input pulley on the Rotrex which is lower internal ratio of 10 or 9:1.
You wouldn't bother with a transmission system on a twinscrew because they are positive displacement blowers... meaning they move amounts of air even at idle... they acheieve max boost by ~2500 on the Eurosport units we've driven. This means that 8 or 9 psi comes on at 2500 and stays there til redline. This makes the car handle more like a big block V8 than a 3.0L inline 6.
Also, you wouldn't really need more than 2 gearings... because either the supercharger is making good boost or its not. The nature of a CF blower is that theres little boost below 3000 rpm and max boost at redline... you wouldn't want 5 gears swapping around because lets say gear 1 is from idle til 2500, gear two comes on at 2600... well what happens at 2550? Not to mention, if you really want your boost to be present throughout the entire RPM range, you buy a twin screw or a turbo
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