Just spoke with my better half about this,as she has worked in the industry for many years and she said that the reason is that they may no longer be able to secure a car as each dealer will be allotted a set number of cars. If they haven't been given the colour you want they may be able to swap there car for one at another dealer. However this would require the other dealer to want the colour they are offering and want to do the swap, they might be wanting to hang onto what they have if the cars are rare and in demand.
As to supply a car you will probably find in the contract that there will be a clause about subject to availability. While the salesman would have assured you that there was one available and just sign here, the reality may be very different and now the hard sell will start with" I can't get you that car but I have this one that I can do for the same price".
The bottom line is if they can't supply the car they will have to refund the deposit, doesn't help you as you want the car.