Sma voting is such a hassle. You have to watch a fuck ton of ads to get a ticket of vote. Voting for one group is already exhausting how much more voting for a few more. So no they did that because they have their own dedicated fandoms.
I've no idea how the system worked, the fact that jumped out to me was that YG groups topped all the categories mentioned in the OP.
Which is odd however you regard it.
Basically it means that fans of all YG fandoms are all bigger and more active than all other fandoms for all categories - at least regarding this voting poll - or that only the categories were quoted in the OP where an YG group topped the list, but that there are other categories where other groups rank first.