I think its the nature of an Auction website.  Ebay is known as an AUCTION site, people come here in general for the auctions.

BIN probably puts buyers off - with an auction they have the perception that they can get the item for cheaper (even though they might not).   People also like to play - auctions are fun, whereas BIN ends the buzz of being on Ebay quickly.

I think it also depends on the type of items you sell - some will do better in auctions than others. 

I used to use auctions and found the my items always went much cheaper with them - they do better in general with BIN. They take a little while longer to sell, but they still sell at the price I want.