How do you know that ? Was there an 11th century population census conducted in the Byzantine Empire that i am not aware of? Did the Byzantine officials go house to house trough the rocky, mountainous terrain of the Balkans or Eastern Anatolia for example to ask people whether they were fluent in Greek or not? Most of the relevant persons in the empire surely must have spoken Greek, but then the vast majority that lived in the Empire during Basil II's time were illiterate peasants. Remember that we are talking about a medieval state that lacked modern communications and stretched from Italy to Iraq.
Consider this: even at the start of the 20th century a large percentage of the inhabitants of Northern Greece (below the Vardar obviously) still spoke Albanian and Vlach... both populations had lived there for the entire duration of "Byzantine" rule. Wouldn't they have adopted Greek earlier if the Byzantine Empire had been a Greek/Hellenistic Empire?
Because the vast majority of writing of any sort made in specific portions of the Empire, from commercial records to bureaucratic stores to church records in obscure villages, changes what language its written in based on what the language of the administration
within the province/theme/etc is done. Most bureaucratic and official records change over first, usually a relatively short time after conquest (mostly pretty much just whenever things have calmed down enough for some kind of civilian government to get going); the central government spoke Greek, so the provincial ones better damned well do it, too. Church records change over next as areas where previously the central Orthodox Church had not been able to reach properly that might keep records in native tongue start keeping parallel records in Greek.
Commercial records turnover last, oddly enough. The slow change here probably has something to do with the extreme backwardness of most areas when (re-)conquered. Serfs have little need of commercial exchange, and the Byzantine economy had collapsed outside Anatolia and the cities by the time period we're talking about.
But what about the commons, you say? Well, all these records certainly don't paint a very complete picture, true, but they're what we have on that front. Fortunately, Byzantium
was a literate culture, with very wide spread literacy for the time. A lot was written throughout the middle Byzantine period I'm talking about, and we get attestations of certain languages being spoken in certain areas less and less over time until they simply stop. While I'm sure even when written testimonials of a language's presence stop that doesn't mean there isn't still a large number of native speakers, that also doesn't mean they weren't in a minority by the point we're talking about.
So, can I say with 100% certainty when and where certain languages died or stopped being spoken in a certain area? No, of course not, but I can generally guess a time period in which usage was declining and make general estimates of when it became a minority.
The areas where the majority of the population was living tended to be Greek speaking. I'm not saying there was a general majority of Greeks everywhere, but the weight of the population of the empire was concentrated in those areas which spoke Greek. That doesn't mean that the entire empire is supposed to be completely 100% culturally 'Greek' or something like that (in fact it's kind of silly to speak of 'Greek', considering the word these people used for themselves was
Roman, which included all the old populations scattered across the area from Epirus to Cappadocia that
just so happened to speak a descendant language of the one spoken in the Aegean a thousand years beforehand; there were no doubt hundreds of sub-cultures within the Greek speaking language area), it just means that the weight of population
spoke the medieval form of Koine Greek. They were probably a distinct minority in areas like Armenia or Bulgaria, but not in Philadelphia or Iconium.
. He asked for Christian Turks.
