Quite simply
The A Series devices that will be updated to One UI7 are newer and more recent devices.
It has always been the case with mobile phones and tablets that they are only updated for a certain period due to advancements on technology.
When the A71 was released in 2020, it was eligible for 3 OS updates and it has had those. Other than possibly the odd Security Update (and that is unlikely now) it will not get any further updates.
Some more recent A Series devices will get 5 or even 6 years of updates but that will not be retrospective for your device.
Time to think about upgrading....
S24 Ultra