The most technical reason for not offering an upgrade to all 10.5 users is: PowerPC-Architecture! 10.6 was the first Intel-Only OS X, and 10.5 the last one for PowerPCs.
As someone who is using Snow Leopard (10.6) on some older macs. The path to upgrading seems to be through apple support or buying a snow leopard disc.
(as the app store offers me mountain lion upgrades on machines that don't support it).
