Growing up. Life experiences of myself and friends.
Kids are shitty and form cliques. Frequently. If your opinion or (perish the thought) your appearance were to differ - it makes you a far bigger target. Taking something to increase that difference will result in more of that behavior.
On blockers:
I won't disagree that blockers are a gentler approach but they are not a magic bullet and do actually run the risk of lasting effects. Very little long term research has been done outside of animals and frankly I don't think testing on children is the best way to go about it.
Much like steroids or any other drug you can absolutely stop if you don't think it's for you but what of the physical changes that occur in the meantime? Side effects are known and listed on even the mayo clinic website. Ill reiterate: why are we letting children take on these additional risks as if they don't exist?