I'm not sure if this is the reason you can't, but note that both of those questions have answers. Community Wiki on a question affects all answers too (from that point forward), so it makes sense to me that the asker of a question shouldn't be able to force that change on answerers unilaterally. Making this change shifts the functional "ownership" of a post away from its author and also denies further reputation changes, which really only makes sense if a bunch of people are going to be sharing in the writing/editing of a post.
Community wiki is best used from the start, to allow the community to assemble a single canonical answer to a question. Its benefit is less clear to me when a question already has a bunch of answers. Community wiki is pretty rare, and Stack Exchange has been discouraging it for the last few years.
That said, moderators can convert a question (and thus its answers) to community wiki. We would want to see clear consensus from the community on any specific proposal first. For these two, I suggest you add one answer for each explaining why that question should be CW, so the community can vote on your proposal. For future cases, just ask a new question, like people do when proposing that something be closed or reopened.