if you look at Boeing's figures http://active.boeing.com/commercial/orders/userdefinedselection.cfm?flash=y, you notice that there were 389 deliveries for the B737-500, the first one on February 28, 1990, the last one on July 21, 1999. For the 737-600 you have up to now (last delivery October 17, 2003) only 51 deliveries and 72 orders, the first delivery being on September 18, 1998. The -500 was for about 10 years in production and they sold 389 units, for the -600 you have only 51 units after 5 years of production. Overall the -600 seems to do worse than the -500, although it is its direct successor and the direct competitor from Airbus (A318) was not available until very recent. Why is that?