Stuck on 5.4.3.3

I see mention of versions 5.4.4.6, 5.4.4.7, 5.4.4.8.
Other devices in my household are on the latter, but my Asus (Android 4.2.2) is stuck on 5.4.3.3, and no updates are shown as available.
Any idea why this is?

EDIT. Just noticed the min. specs have changed to Android 5. Great. :-(