this is a silly question. I suspect those not getting into their “target” schools is because every one and their neighbour is applying to the same top 5 name schools so plenty of competition for a limited number of spots. 

luck of the draw

First I'd say the correct word is rejected. Denied makes it sound like someone had a personal grudge against you and blacklisted you before you even applied... highly unlikely. 

As for why, you didn't fit what they were looking for this year. That's what it all comes down to plain and simple. You may not be up to the standard one school is looking for in an easily quantifiable way, or it may just be a matter of preference for the design stylings of other candidates by the people reviewing portfolios. It's really hard to say. This is why sometimes people will get accepted into school they really don't expect to but not ones that seem like slam dunks. It can go both ways.

From the perspective of "target" schools, they might be cautious that some exceptional candidates have them as "targets" and not "reaches". If a candidate is so good that admissions feels he or she would probably be admitted by the "reaches", then the "target" might be disinclined to pick this applicant ahead of one who might actually choose to go to the "target" in question. GSAPP used to have this problem - they are the de facto safety school for international applicants to the GSD and SOA. the top notch applicants will not go to GSAPP if accepted to the other two.

My personal opinions, and I am no expert. I think it might be similar to UG admissions, where the applicant pool from a singular UG program is kind of compared. One of the programs told us, they like accepting applicants from different backgrounds. There are also design differences, where 1 person's style may be a better match for a program than another.  Maybe the programs place emphasis on various parts of the application, like employment or who the reference letter is from.  I really would not waste too many brain cells.  You will end up where you were meant to be. I am also wondering with some schools, if they like to take 3 yr applicants over 2 years, because they can train them how they like, instead of getting applicants who have to unlearn.