-convenience -because it would cause mass-hysteria among the humans if they knew vampires were real. -as mentioned, humans could possibly kill all the vampires if they put enough money into it.
Charlie doesn't know that The Cullens are vampires, but he do know that they're not normal, but he don't want to know what they are.
A secret
all rich men are commonly misers so they want to keep this as a very huge secret.
If a company is profitable they may want to keep their marketing a secret. If a company markets their company in unusual ways, they may want to keep it a secret.
We don't know, they're a secret.
This riddle describes a secret. If you want to keep a secret, you must keep it to yourself and not share it with others. However, once you share the secret, it is no longer just yours, and the essence of it being a secret is lost. Thus, sharing a secret leads to its disappearance.
What big seceret did Mr. and Mrs.Dursley want to keep
Well, if you liked a guy what would you want him to do for you - tell or keep it secret? What you would want in the same circumstances is probably what you should do.
No, she did not keep her methods a secret.
Women like to keep their weight a secret because they don't want people to make fun of them if they are a little overweight.
You don't tell anyone.
Bella wanted to keep the fact that her transforming into a vampire was happening sooner than Edward had anticipated a secret from him.