driver compatibility.
Additionally, in practice a module (driver) that ignores the kABI will usually work for all patched kernel versions on a given minor release (~service pack). e.g. A minor release 6.X introduces a new kernel X' (2.6.32-X'.Y) and a module built against X'-0 will work for all Y.
(CentOS/RHEL 7.x is new enough that this isn't a big issue yet, but it normally does become a pain point the older the RHEL/CentOS major release gets - there are a lot of drivers that does not get backported to the RHEL/CentOS minor releases)