Apple can sell content as they received it from the supplying studio for your country or country-like region. When Apple receives a 1080p HD edition for the AU Store, then that is what they can sell to end-users there. For 4K editions, do ask the supplying studio to submit a new UHD edition to Apple. For the mentioned movie, that appears to be Universal, although three studios are mentioned. Distribution rights may have been sold to one of these or a different company for your region. I think Universal had the ‘international’ (non-US) distribution rights initially. Try their social media presence for consumer questions.
Unfortunately, releases are not the same worldwide, but divided by country or region.
Gladiator (2000) on iTunes - Apple (AU)