My best guess is that they may be receiving many responses. We have no idea how many people are trying to rent to them. They are either overwhelmed with replies and have gotten something already, or they are more likely to post a request than to actually book something.
I would probably think they already booked with someone and responding to all the other requests is too much work for them. Imagine getting one hundred replies. Would you respond to all the ones you didn't book with?
Just a thought.
Btw, I checked my spam filters and if anything it gets sent to a junk folder and there were no emails there. I have only received 2 replies from responses I sent, and have sent out at least a dozen responses including a voice mail to one request with an outside email. When I get to hundreds someday, I will update with my findings.
Ronald L.