Honestly, it seems to me you gave up pretty easily there.
High barrier to entry. Maybe you think the barrier to entry for Zabbix is low enough, and maybe you think it is a better project once you get over that bar, but what is undeniable is that Graphite very clearly has a lower barrier to entry.
If you want to know why so many people use Graphite instead of Zabbix, that is the reason. If you want to tell all of those people that they are wrong for going that route, be my guest, but that isn't what the discussion is about.
I've answered your question, I feel, entirely accurately.