When someone expects something out of you, and tells you explicitly, there is the chance for the moderate solution that can be described as “the polite no”. When they explicitly tell you that they want something, and for some reason you are either unable or unwilling to give that to them, you can politely refuse and reason out with them as to why you are unable to give them what they want.
On the other hand, if someone expects something out of you but doesn’t explicitly tell you that, that removes the possibility for this beautiful intermediate solution. Firstly there is the problem of guessing – you’ll have to somehow divinely guess that that is what they want. Secondly, if you fail to deliver, there is always the possibility that they ASSUME that you were UNWILLING to deliver, even if the reason you didn’t deliver was that you were UNABLE to do so. And this might lead to some sort of bad blood, or more misunderstanding.
Still, I don’t understand why some people never state what they want out of you. As to why they are being cryptic than being direct. I don’t know if they realize that they are ultimately putting themselves at a loss by means of this lack of communication.