TASP is so prestigious because the acceptance rate is 3%. Further than that, the reason the rate is so low is because the admissions committee specifically looks for candidates who they believe are, essentially, special snowflakes. They go beyond looking at grades and test scores (though most who get in are stellar in these areas) to choose students who they believe are passionate and, above all, interesting human beings. Colleges want people like this, so they consider TASPers to have been already vetted, beyond what university admissions committees can already ascertain from their transcripts.