^^In the case of Greece, which was officially diglossic until 1976, I'd say that the decision to make the Demotic Lower register the official state language, was a political one. The Greek diglossic question acquired from early on a left-right orientation, leading to a language war spanning more than a century, with the Left fighting for the Lower register and the Right defending the Higher register of artificially constructed Katharevousa.
In the sixties, while most of Western Europe enjoyed a liberalization, Greece experienced a resurgence of right-wing conservatism, which culminated in the 7-year military dictatorship of 1967-74. The military regime banned Demotic, claiming it was slang, the language of hippies and communists alike, while Katharevousa was the proper language the Greeks with such a glorious past should use.
In 1976 the democratization process ended the diglossic question by formally adopting the Demotic as the official language of the Greek Republic. But the decades of conflict between the two registers, had irredeemably changed Demotic, which by then had acquired Katharevousa traits and had trimmed away all the folksy, rustic and dialectal words, forms and syntax.
To give you an example of the absurdity of Greek diglossia:
My mother (born in 1946) in her gymnasium years (middle school) took Ancient Greek. In the ancient language,
fish is
«ἰχθύς» ĭkʰtʰū́s (masc.).
On the other hand, she spoke Demotic at home, so she naturally called
«ιχθύς»,
«ψάρι» [ˈp͡sa.ɾi] (neut.)
*.
At school though, she could use neither ἰχθύς (too old), nor ψάρι (too slangy), so she was forced to use the Katharevousa's
«ὀψάριον» [ɔˈp͡sa.ɾi.ɔn] (neut.) instead
*MoGr
«ψάρι» [ˈp͡sa.ɾi] (neut.) -->
fish, aphetic form of Byz.Gr. neut. diminutive
«ὀψάριον» opsárion -->
fish < Classical neut. noun
«ὄψον» ópsŏn -->
side dish; the name «ὄψον» became gradually synonymous with the
fish-dish and eventually with
fish in general (PIE *h₁op-s-
side-food cf Lat. opsōnium,
anything eaten with bread to give it relish, especially fish).