Easy answer:
When the wings are swept back the airflow is accelerated less as it flows over the wing. Only the component of the airflow perpendicular to the wing is actually accelerated at all. Therefore, a swept wing will have a proportionally higher critical mach number.
More complex answer:
Mcrit is the lowest mach number at which the airflow at any point on an aircraft becomes sonic (mach1). The speed of airflow at any point on an aircraft is the freestream speed plus whatever acceleration the aircraft has given to it. So Mcrit is mach 1 minus the greatest acceleration found at any point on the aircraft. If for example the greatest acceleration is mach 0.2 then Mcrit is 1 - 0.2 = 0.8. So if we can reduce this acceleration we will increase Mcrit.
The critical Mach number is not the absolute Mach number, it is the local Mach No, perpendicular to the leading edge of the wing. So, if you resolve the triangle of velocities, for the "ambient" Mach number to be that parallel with the wing centreline, then we get a lower apparent Mach number perpendicular to the wing leading edge. (It's a bit like calculating crosswind component). So, the greater the sweep, the lower the perpendicular Mach number for the same flightpath Mach number.
TP