This question is about the policy for general contest problems, NOT ongoing contest problems.
If it is because the question is too localized/ad-hoc/without motivation, I would argue that the same happens for many modern contest problems. Are all such problems going to be closed? Also, it seems to me that many computational calculus problems are no better motivated than a problem like this. (e.g. whether a random series converges, or how to compute a random integral, or whether a sequence is monotone) So this does not seem to be a sound reason to me.
If it is because the OP did not show enough effort, as commented by user45878 in the second post, then I wonder what level of effort is sufficient. While it is reasonable to say that OP did not show enough of his work in either posts, olympiad problems are often tricky enough that if you don't get the idea, you are nowhere close to solve it at all and it could be difficult to get any partial progress. Even though the question is labelled as homework, it seems harsh to demand the same effort level shown compared to a standard college homework problem.
It would be immensely helpful if others can shed light on this, especially users who closed the mentioned questions themselves.