Yeah, POs are notriously unresponsive about why they are unresponsive. I agree with darkwingbuck's answer.
I think PO's get a lot of queries from newly qualified mongers, undecided "looky loos" thinking about taking the plunge. (Most of aampreview readers are probably looky loos, maybe never actually mongered.) Queries, like whose available, etc. The POs don't want to list all available, then have the newbie monger, go and say I'll think about it. Too much work for nothing. Why should PO be polite and answer "not available" if it encourages a "who else?" dialog, with an unlikely to get a booking.