I find it hard to explain it simply, maybe there is a way but I do not know it, I spent some time getting my head around the vectors and the t/c ratio and relative camber.
The best way I could explain it is that on a straight wing, all the air hitting the wing is going straight, chord-wise backwards at a certain velocity.
On a swept wing, part of the air is going chordwise, and part of it is going span-wise. The span-wise flow uses some of the energy that would be in the chord-wise flow on a straight wing, so if energy is removed then the chord-wise speed will decrease, so free stream mach no. must increase to get more energy to increase the chord-wise flow velocity back to sonic.
I know this isn't the most correct or scientific explanation but it is one that I find simple and has enough to it to suffice.