Article: Due to the centralisation of power and the Viking invasions, there is relatively little written record of the non-Wessex dialects after Alfred's unification. Some Mercian texts continued to be written, however, and the influence of Mercian is apparent in some of the translations produced under Alfred's programme, many of which were produced by Mercian scholars. Other dialects certainly continued to be spoken, as is evidenced by the continued variation between their successors in Middle and Modern English. In fact, what would become the standard forms of Middle English and of Modern English are descended from Mercian rather than West Saxon, while Scots developed from the Northumbrian dialect. It was once claimed that, owing to its position at the heart of the Kingdom of Wessex, the relics of Anglo-Saxon accent, idiom and vocabulary were best preserved in the dialect of Somerset.

Now answer this question: Along with centralization, what reason is given for the lack of records in non-Wessex dialects after the unification of Alfred?
the Viking invasions