I don't think undergraduate STEM is the reason. If you look at the
incoming salaries for Tepper, it's actually pretty low compared to other business schools. The STEM should affect the incoming salary, rather than the post-MBA salary.
A lot of the ranking has to do with qualitative assessments of the program - how peers and recruiters view the school. I call this the "let me pick a random number from the clouds" aspect of rankings. Furthermore, Tepper's selectivity of students is lower in terms of GPA, GMAT and yield rates. But in any event, Tepper sure has great success in job placements