The SAT has been dumbed down twice, first in 1994 and now again in 2016. Perfect scores in either section were rare prior to 1995, but particularly in Critical Reading. Back then it was the Scholastic Aptitude Test and was a sufficiently good test of IQ that Mensa accepted it for admission. Now it is much more of a knowledge test that can be prepped for quite easily.
As to why it has become easy, you can think of it optimistically or cynically. Optimistically, you could say that colleges found that scores beyond 750 showed no difference in how successful a student would be at college. Cynically, you could say that certain demographic groups were dominating the scores at the high and low end and compressing the score ranges allowed selective colleges more freedom in holistic admissions.