Due to the DI system and lack of carrier rails an AR platform at high rate of fire may simply allow too much crap to build up in a way that interferes with bolt/carrier function.
In my opinion, the reliability advantages of the AK are its industrial-strength dump truck of a magazine vs. the delicate and intended-to-be-disposable USGI aluminum mags the AR is often saddled with (newer mag designs narrow that gap a bit); the AK has a somewhat longer stroke and uses a slightly shorter round, so the bolt carrier can get a bit of a running start before hitting the top cartridge in the magazine; and its more open interior design and "floaty" bolt carrier are more tolerant of large debris than the AR's cylindrical receiver is. The very tapered round also gives some theoretical feeding and extraction advantages, at the cost of a very curvy magazine and a short magwell.
On the other hand, the AR does a pretty good job of keeping debris out when buttoned up if you use magazines that fill the magwell (not aluminum USGI); the AK is more tolerant of debris in the receiver but doesn't button up as tight, and I suspect that indigenous AK users in the middle east wrap their rifles in sandstorms and clean them after just like M16/M4 users.
I don't think DI has all that much to do with it, honestly; I don't see good DI AR's being any less reliable than piston-upper AR's in 5.56mm with normal-length barrels, although pistons definitely have an edge with very short barrels and sound suppressors due to pressure/timing issues.
As far as fouling, unless you're running a sound suppressor, in my observation an AK and an AR get comparably dirty from similar round count. The AR
vents most of its gas to the atmosphere after bolt carrier launch just like the AK, and the AR puts a bit of gas into the receiver from case blowby during extraction, also just like
the AK does. The AR gas tube also adds a very small amount of gas into the receiver from residual gas tube flow during extraction, but this is not enough to make much of a fouling difference in an unsuppressed gun.
IMO, powder residue is rarely a cause of jams in a properly put together weapon, if some lubricant is squirted in there
every thousand rounds or so and the rifle started out lubricated. Foreign debris/contamination is far more likely to jam both designs than powder residue is.
If the ARs DI system were not so problematic, or at least equal to piston driven, why are there no new guns being designed or built with it?
A good question. I'd venture it's the case because any DI gun that's not an AR is going to have a very hard time competing against the AR. The AR's DI system has been refined by a half-century of trial and error, the system is already modular enough to become anything from a pistol to a big-bore rifle, and the Stoner design has the imprimatur of 5 decades of military service, so there simply isn't much incentive to reinvent that particular wheel. On the other hand, piston guns offer a means to differentiate a new gun from the Stoner design and create marketing talking points, so piston designs probably have a bit better chance of being a viable non-AR alternative in the market.
Since the evolved Stoner design is the longest serving and most successful infantry rifle the USA has ever fielded
and is the top selling centerfire rifle in the United States, I don't know that lack of DI replacements for it is necessarily an argument against the merits of the platform. True, it's not being superseded by another DI, but it's not being superseded by piston guns either, even though plenty have tried.
I do think the FN SCAR-H/Mk 17 offers definite advantages over 7.62mm AR's, though, but the latter have been far less refined over the years than 5.56mm AR's have.
Also, why are no new guns being designed with a carrier that travels through a tight tube?
Because only a DI gun is practical with that receiver design, and no one is making new DI designs because they wouldn't be competitive against the AR.
You could certainly do DI with rails if you wanted, but it's very hard to do non-DI
without rails; a piston gun can't avoid off-axis torque, so piston guns require carrier rails to prevent tilt.
. 
Like you said, it gets back to getting hands on the rifles, and getting some experience with both. 
). Ergonomically, I really like the cheek weld from the saddle (I guess that's the best way to describe it) of the original Russian style stock. It's a good rifle. Right now I'm more comfortable with my M4geries because I've just spent so much time with AR type rifles. AKs are definitely good rifles though.
