SBL/AARdvent Calendar: Day 1

A question for Bart Ehrman:
What do you think is the best reason for rejecting the notion that Matthew used Luke?

The attached screen grab (from the Bart Ehrman Blog) shows a blog member, Tony, asking a related question back in December 2017. Bart's answer is noteworthy because he only states why it is unlikely that Luke used Matthew. This is not, however, the same as offering a reason why Matthew could not have used Luke. The fact that Bart seems not to have an answer to this question is striking given that, less than a week earlier, he stated that he found Mark Goodacre's rebuttal of Matthew's use of Luke completely convincing. I would find it  very helpful if he were able to articulate, in his own words, why he thinks Matthew could not have used Luke.