With the dawn of Protestantism in Britain, the wide use of Latin in writing began to fall away. This can be understood in terms of an attempt to put distance between the new faith and Rome. It would make religion more accessible to the normal citizen, rather than them having to rely on churchmen to explain religious teachings.
Under both Mary and James, literary life was encouraged at court. James himself wrote and traslated poetry. At the age of 19 he wrote a set of standards for the writing of poetry in Scots.
But at the same time there was increased use of English at court. This spread to wider society.
Knox had written in English and this was taken up not only by churchmen, but by poets, playwrights and authors of the time. However, not many Scots were able to understand English either.