There is a fairly obvious explanation for that, in that students in upper level courses are usually majoring in the subject, hence taking the courses of their interest, rather than exploring various subjects or taking general education courses that may not be of their interest when they are frosh/soph students taking lower level courses. Also, at schools where some majors are capacity-restricted, only the stronger students from the lower level courses are admitted to the major to take upper level courses in those majors.
In the sciences, where prerequisite requirements tend to be important, a student whose knowledge of the lower level material is not up to par will have difficulty with the upper level material. So the elitist doubting of a science student who performed well in upper level courses in his/her major due to having taken lower level courses at a community college makes no sense. And that likely applies to many other subjects besides science ones.