Facelifts are all about marketing.
If they let the Leon run its life for 6 years as it is what's the appeal to upgrade to a new one?
If they give it a facelift then at least you're upgrading to something that's a bit different to what you had before.
Also they can advertise it as the 'new' SEAT Leon, which will
help it complete with other new and facelifted models.