The reason is because AMCAS changed their traffic guidelines. Apparently now students are allowed to hold multiple acceptances but must
confirm to enroll by a certain time. There is a thread on the different dates for the different programs in which all applicants have to confirm to enroll by:
2018-2019 "Commit to Enroll" Deadlines
The reason this sucks is that since it is not enforced and use a guideline many individuals could hold these acceptances until the confirm to enroll deadline. If you go to the link there are some schools which have this deadline until MID-JULY

Since the MD schools will have little movement until the confirm to enroll deadlines where students will finally scramble to make their final decision, there will be little DO movement, because many people do prefer MD over DO.
Which is why, it is important, and I cannot stress this enough: IF YOU HAVE BEEN ACCEPTED BY A SCHOOL REMOVE YOURSELF FROM OTHER WAIT LISTS.