I was there on Fri and the ground was not just boggy. The 1-2 inches of surface water was flowing downhill in a mini river. IMHO the suggestion of having alternate car parks was only half the story. The stands, stalls and food outlets on the grass would also have been inaccessible to the vast majority of normal people (obviously Glastonbury diehards would have been in their element). As most of the stands have some kind of tented/vehicle based coverage to prevent damage by rain it would not have been practical to move them onto the handstandings (always assuming there wa room).
Even if there had been alternate car parking, i think there would still have been complaints about the lack of stalls etc; in short, the organisers were caught between a rock and a hard place by exceptional weather.