In mid-2012 Leopard users were offered a free DVD upgrade to Snow Leopard to get the App Store (and access to later upgrades).

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.

The question isn't why they aren't offering upgrades to all 10.5 users, it's why they're not offering upgrades to any 10.5 users.

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).

No app store in Leopard? If the MAS is the only distribution vector, there's a bootstrapping problem from 10.5 IIRC.

10.5 doesn't support the Mac App Store. And, why would they put effort into testing the upgrade process from an OS that is only still used by people who never upgrade the OS?

snow leopard was the first 64-bit only OS X. You couldn't allow all Leopard users to upgrade, so that's not what's in the press release. People may still be able to upgrade though, if they can work out how to install the App Store.

Snow Leopard still supported 32-bit processors. Lion requires a 64-bit processor, and Mountain Lion requires 64-bit firmware.

apologies - I remembered it wrong!