Its the lines that awd cars make and the speed at which they take them. To tune an awd drive car to handle like an fr is going to result in higher entry and exit speed on an fr drift line; to maintain the awd tune means an altered drift line that will intercept other fr vehicles...is it impossible? No, but it takes driving skill and tuning prowess, that frankly many that insist on using awd to drift socially don't possess.. I do it from time to time, usually with any manner of imprezas or evos, but it's a crutch; it doesn't encourage the development of good drift skills.
This is the best answer so far.
The "crutch" effect of a fast 4WD car means the drivers do not have to improve to keep up. The same can be said of drifters who use any significantly faster drift car against lower-speed but higher skilled drivers. It is just more common with 4WD drifters for these reasons. Gymkhana, rally, what the car is "built for" or whatever doesn't mean **** to me but the only real difference between 4WD and FR cars is what happens when you apply throttle. And somewhere in that throttle application are reasons why the drifters tend to be bad. I'll give a quick explanation here on my theory on it, for those that care: (for those that don't, just skip to the end)
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There are generally two types of 4WD drifters, (or drifters in general) the ones who enter a corner gently and naturally start drifting without throttle, like any RWD car, displaying balance and optimizing their angle and speed, and then maintain with throttle with barely any counter-steer and exit with throttle with high traction. These have general similar lines to RWD cars, except for the higher entry and exit speed. This style is fine and easy to adapt to in tandems.
However, the other 4WD drifters (and many RWD drifters) just use throttle on entry, mid corner, and exit with little or no careful line selection, or, throw their car in with the E-brake and then slam the gas to straighten it out. These are both easy to do because in a 4WD car accelleration on the front wheels introduces understeer that pulls the car out of the corner without the need to countersteer further as throttle is increased, unlike a RWD car which will lose spin and spin out, hamfisted gas-slamming actually speeds a 4WD car up coming out of a slide. That, and enhanced forward moving traction on low grip tires, means that they get lots of additional speed in the exits and straights, and can regain traction from silly entry speeds easier than a RWD car can.
What's important is that in RWD drifting, is that those same techniques result in significantly less speed and control through the corner. Slower cornering and more wrecks causes a drifter to significantly fall behind other drifters and not be able to tandem. It is a sign that the driver needs to improve their control.
But in 4WD cars the benefit of higher entry, exits, and straight line speed more than outweighs this. This means that a poor 4WD drifter can "keep up" with a good RWD drifter. It is very much a "crutch" as it doesn't indicate that a drifter needs to improve their skill to maintain enough closeness to "tandem", despite the sloppy driving and poor overall control.
In other words, the problem with 4WD drifting is that 4WD drifters are disproportionally unskilled compared to RWD drifters, because they are, when drifting with RWD drifters, never really required to improve to keep up. Since 4WD drifting doesn't have a very strong competitive group or following on GT5, the amount of poor 4WD drifters who use the added speed as, as you say, a crutch. Result: More unskilled 4WD drifters, making 4WD drifters overall less pleasant.
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On the subject of cars I drift, In the past, the best 4WD drift cars I have used are, in descending order of ability:
Best:
22B Impreza (never the four door imprezas! Only the 22B!)
Evo VI T.M.
Evo II
Mine's BNR34 N1 (S or P)
Nissan R32 Skyline
Toyota Celica GT-Four (The '98 Model)
Mine's Evo VI
Ford RS200 '84
Peugot 205 Turbo 16
Every other Group B Car
Every WRC Car
Golf R32/Audi A3/Audi TT/Any other Haldex FF to 4WD car
Every car not yet mentioned.
The Audi R8 gets honorable mention as just a very, very good car overall.
I don't drift 4WD anymore, but I might break out the 22B some day again... it's uncompromisingly savage, but not near as fun as the 240Z.
