Since this is "chaff," I'll posit the following: It's so we can order only one or two items at a time and thus keep the US Postal Service solvent! :-)
I understand BPL's not wanting to have too much inventory on hand (for a small business, it's a big overhead cost!), but it would be nice if they could keep a more constant stock of the really popular items. Since some of these popular items seem to be specially made for BPL, this may not be possible. It is frustrating for us potential buyers, though!