over the last 4 years, apple hasn't given away major updates: 8.x to 8.5/6, for example, and 8.6 to 9.x; 9.x to 10.0, and 10.0 to 10.1. However, going to 10.1 from 10.0 was available on an update CD for $20 or free from many Mac retailers.
8.6 is old (but a good release) as far as Apple is concerned. Their focus is on OS X (and now 10.2)--it shouldn't be a surprise (thought it is a disappointment) that 9.x isn't sold separately.