You mean the staff car park? Coach park?
There is no room and they probably when building the car parks were told they could only put it in that area... So that's where it is... Towers is certainly a place that demonstrates the evolution of a park over time, where they had to design things along with the park's own evolution... It is simply not possible for them to have car park space of the size required to be at the park's entrance (hence the rumours of the second entrance by Valley, but I find that idea silly really)...
The Monorail is a lovely way of entering the park, if they could improve the throughput of it on busier days it would be a lot better (and prevent queues to the toilets, which are rather off-putting and result in walking)...