Yes I believe it is because the increasing number of Platinum users is beyond their expectation.
On the other side, they have limited number of VIP host while it is not easy to hire people that can be trusted and educated to be VIP host.
I got VIP host when I was ranked up to Platinum 1 few years back and I still have it till now, so lucky me to start earlier
If the number of Platinum users (1-5) is increased significantly but Stake is unable to add more VIP hosts, the requirement might be increased again as what Mahdikarib said.
I am very far to become platinum rank on Stake and will never reach it, but why would it be difficult to educate people to become VIP host? You just need to have a good knowledge over the site and can handle rude customers. In my eyes, it doesn't look so difficult but maybe I am overseeing things...