Because it was the most Christian civilisation over many generations (about a thousand years).
The above is the answer.
If you are not a Christian, presumably you would not agree that this was the greatest civilisation - since you are looking for completely different attainments (military success, peace, comfort, artistic or philosophical attainment - or whatever it might be).
If you do not understand Christianity (e.g. if you are not yourself a Christian), you would presumably disagree.
Or, if your understanding of Christianity is significantly different from how Christianity understood itself for the first half of its existence, then you would presumably disagree.
Or maybe you don't know anything about the subject of Byzantium - like most people.
Or maybe you have read only/ mostly secular historians on the subject of Byzantium, or an account actively hostile to Christianity (this was my situation into just a few years ago)...
...then you would probably be ignorant of the Christian-centred perspective on the civilisation, and unable to make an informed judgement on the matter.
But for someone with a broadly 'catholic' understanding of Christianity; then Byzantium was the apex of what man has achieved so far - and almost certainly the apex of what he ever shall achieve (on earth, in this life).