Is it because it doesn't start with a capital letter? There's a perfectly reasonable answer with 5 upvotes. The question is not off-topic nor is it unclear to my knowledge.
The problem is that the question is unclear, and the author never improved the question. Someone edited the title to add in the capitalization, and it still is unclear. The "perfectly reasonable answer" that you mention strictly speaking does not answer the question, because the question is asking about DC power (and a crystal radio doesn't generate DC).
You say that it is clear to your knowledge, but there are several bits of information that were not included that would make the question more answerable. You could make some guesses and come up with an answer, but then you're not necessarily answering the question that was intended. Below are a few pieces of information that would be necessary to get started on an answer:
- How much power are you trying to collect?
- How far away is the transmitter?
- How much power is the transmitter emitting (or what is the field strength at your device?)
- What frequency are you using?
- Do you control the signal that the transmitter is sending?
- What kind of antenna does your receiving device have?
You should only ask practical, answerable questions based on actual problems that you face. Chatty, open-ended questions diminish the usefulness of our site and push other questions off the front page.
Because this question is a hypothetical question with no application, the asker is unable to fill in any blanks and we are left with just a yes/no question of
Is it possible to convert RF to DC?.
Its a hypothetical with no constraints listed. If it doesn't violate physics, of course its possible. With real specs and constraints, it can become a real question. Until those are added, its not. I'm content to let those go by. If it were more interesting, I might have asked for more info. Its not something I'd vote to reopen without a major change.