Brown is a selective university for the same reason other universities are selective. There are significantly more extremely qualified applicants who want to study there than there are spaces available.
There is a thread on this, just four or so below yours. At last count, 98 posts. Upon perusal, if you still have some unanswered questions maybe you can post them there.
http://talk.collegeconfidential.com/brown-university/1725236-brown-rankings-p1.html