The CDC's own data posted on their own website indicates MMR rates have been constant for 20 years.
what has gone up is the number of vaccine exemptions. A child is considered to have an exemption if he or she doesn't have every vaccine in the series. As more Vaccines are added to the schedule, children will miss one or another. So the total number of exemptions is monotonically increasing as new vaccines are added to the schedule, but the rate of vaccination with MMR, which is all that's important for measles, is staying constant.