There is no Skyteam rule that requires airlines to offer discount upgrades to Skyteam Elite and Skyteam Elite Plus members.

Just because a carrier like Delta will stuff business class absolutely full with its own elite members doesn't mean that this is a Skyteam requirement. US airlines have a different outlook and seem to like "rewarding" their frequent fliers with upgrades. Outside of the US, most carriers take an opposite view - that the business cabin is a refuge and is not to be packed full with free upgrades. As others have noted, most airlines do not want to teach their frequent flyers to buy cheap with the expectation of getting an upgrade for free on top.

Air Europa used to have a paid upgrade bidding system in conjunction with Optiontown, but it appears this was discontinued some time ago.