I'm not sure where you're getting that from. I've read a lot of data, eg the link below, that shows that engineers do very well, but aren't the top major.

With that in mind, it's not as if all students from all majors are required to take the exam- the data represents those people that self-selected to take the lsat.

http://www.potsdam.edu/academics/AAS/Phil/upload/LSAT-Scores...