Why are multiple kernel versions maintained?

From: ghost of filth (unh_at_ly.org)
Date: 12/23/03 

Date: Tue, 23 Dec 2003 16:41:56 GMT

For example, why would someone remain following
the 2.4 (branch?) while there is 2.5 and now 2.6?

Is the eventual abandonment of a given kernel version
(e.g., 2.0) timed with hardware obsolescense?