Because it's a 2" hole. It's not that it's harder to clean, it's just harder to see inside and it's piece of mind I guess. I'm sure a carboy brush would work wonders. Another issue is that as you add more, you need a sanke tap for each and they're a lot more pricy than corny disconnects.
They're not hard to clean, but you have to rig up some equipment (pump, modified coupler) and use some nasty chemicals to do it right. Cornies allow you to use PBW, which is much safer to handle than hot caustic.
This type of keg is all I use!! Removing the retaining ring and spear is tricky at first but you will get good at it. Clean just as you would a carboy. I typically just rinse w/ hot water, soak w/ oxi-clean, rinse and sanitze w/ star san before filling. I have a carboy type brush if I feel its needed. True, the sankey D coupler is about 25 bucks or about twice the cost of the fittings required for corneys, but the D couplers are very reliable and almost indestructible. I've never heard of someone saying they have 3 gallons of brew in the bottom of their kegerator due to a failed sankey connection. No fiddling w/ o-rings and poppits and lid seals, no need to presurize the keg to seat the seal. Oh, I almost forgot to mention, check valves for liquid and gas are internal to the coupler so you won't be sending product back to your regulator.
IMHO they are superior to corney kegs. They are a little larger though at 9 1/4" dia.