Several reasons really.
1. You do not have enough power. For a AWD car to drift well, it needs enough torque to spin all 4 wheels. So you need lots of torque from the engine, short gear ratios, or a low friction surface.
2. Most AWD cars have a fairly central torque split. Most cars are around 40-50% of power to the front wheels, which causes a lot of understeer. KB's fiesta probably has a 20-30% distribution.
3. Open diffs. You need locked diffs/LSD's or you're just going to piss all the power out through the front wheels. Which also causes understeer.
I'm not saying KB has no talent, he can be pretty good, he's had a few strong finishes when he's not crashed or broken down, but he's even more inconsistent than Yari Matti Latvala used to be, and way more inconsistent than McRae.
And RE drifting a AWD car set up like that being easy, well yes it is. Definitely easier than drifting a RWD car because if you over-angle it you just give it full lock and drop a gear and it WILL come back to you. It is nothing like driving a normal AWD car so you can't relate to it if you've just driven a scooby, chucked it in and booted it, then wondered why you understeered off the road, which is what I imagine Bonxy was thinking.