Why would the public NOT know?

I'm trying to figure out exactly how much the general public would or wouldn't know about mythical creatures existing for my contemporary fantasy. I'm toying with the idea of wizards and warlocks being known, as well as certain supernatural diseases (vampirism and lycanthropy in particular) but I'm iffy on the actual creatures like fairies and kitsune.

Bit of backstory: Most of the mythical creatures were sealed away in a separate world in the early-mid 1800's after a violent war involving Mythics and demons (not the fallen angels/beasts from Hell sort). Ordinary humans were not aware of this. Only a handful remain our world today, usually living in their own areas away from human civilization, but there are some urban Mythics. Because of the low population on Earth (less than 1,000 of each group or type) and the fact that they are very much mortal, Mythics don't want to attract attention or be hunted. So I'm thinking that the governments and law enforcement know about them, agreeing to let them live in peace as long as they don't interfere with humans (there have been incidents, primarily wars that end up involving Mythics to some extent).

But would it be believable that Mythics haven't been revealed to the world under these circumstances? The main thing I need to be secret from general public in the beginning is the other world where most of the Mythics and demons were sealed. Either way, there will be magic humans, Mythics, and half-bloods working with the government and police to keep an eye on the magical population...

Curious, in our world ~ have you heard of "little green men"? Or The Greys? What if it is like that, an open secret, that everyone kind of knows but dissmiss because if it's true the world is suddenly a very scary place, that really isn't safe?

You know what I think could be a fun variation on this idea? If everyone knew. Not about the other world, because it sounds like you have plot reasons for that to be a secret, but that the creatures, diseases and magic users are real.

If it was a major war that caused most of the creatures to be sealed away, I would find it hard to believe that no one noticed. So, rather than making the general public look like a bunch of twits who can't see what's right in front of them, let them see it. Let them know about the war, and believe that most of these magical creatures were simply wiped out in it. There are small pockets of the world where they survive, in what amount to magical nature preserves, but for the most part people just assume they are an endangered species. Why cover up more than is absolutely neccessary?

I agree with the idea of the open secret, let folks know about these things but because of time, ordinary people think of mythical creatures as being in a place (the past) that's far far away and doesn't actually affect them anymore. The fact that this war happened in the 1800s could also help keep the masquerade going, mythics and humans alike start settling in America, Australia and that and the humans think of mythics as relics from the Old Country that couldn't possibly follow them. The government needn't get too involved in cover ups other than planting the idea of mythics as being old superstitions that have no place in an enlightened world, eventually folks would just forget about it.