I've grumbled about gate B a lot in the past - not had the joy in the last two years. I think my grumbles have been outweighed by volume of grumbles for gate D (as suggested, more people use it).
Problems I've experienced at B have often been largely due to late-arrivals joining the queue at different points, and things can spiral out of control with (almost certain) lack of supervision. It can flip things from you arriving at whatever time and being in an established queue, to that queue grinding to a halt as other newer sections of arrivals get preference at pinch points or whatever. Nothing sinister or logical, just people aiming for what they can see. Most of the festival site's parking seems to be on the West, so I can imagine most people experiencing problems are over there, not complicated.
Without great knowledge of the area around D I can see that there are way more potential spots (in comparison to B ) where you could get stuck in one queue that suddenly becomes defunct due to later arrivals doing their own thing, arriving from different directions due to new parking fields opening maybe. I can see addition of a second 'alton-towers' style queue, complained about above, may actually help alleviate the issue I'm describing, but presumably adds other issues as suggested by @fewcloudy
Gate C is obviously different - probably one of the most used entrances but there's only really one way of marching through the campervan fields, so everyone's likely to end up in the same ordered queue - and a huge population of people coming from campervans who a ) don't need (/want) to bring in a massive load of stuff and b ) can look out their window and decide bugger that, I'll sit outside with a beer for a bit.
As for lack of services after getting through the gate - most people don't spend much time there. You could probably make a killing on Wednesday morning, but not exactly a spot I can see vendors lining up for. Once people are done getting their stuff through the gate I can't imagine there's much foot-fall at all. If there's more services going to be put there then the festival would probably need to offer a pretty decent contract.