I wouldn't say 'popular'...in my school only 2 of all sixth form leavers decided to study law, more popular choices included: Maths, Physics, Economics and Engineering (not sure if this is the same everywhere but it's certainly the case where I go to school)
I think because there are no specific subject requirements to study law that they raise the standard a little, and it is a very demanding subject with hours and hours of study time required every week. I think the universities want to see that you are able to study efficiently and accurately and get the best grades.