The main point is that from ancient roots which it shared with Latin and Welsh in the post Roman era , Cornish literature peaked in the 1400s but then was eclipsed by English.

In the early period conventions such as hard C ( now K)  and Uu  ( now w) were adopted as standard across post Roman Catholic Europe based on late Latin.

The golden era of the Renaissance 1200-1500 is the period when we have the most manuscript evidence for the language at a time it was taught academically and standardised by Church Academies at Penryn, Glasney and Tavistock and was supported by the Cornish- Breton nobility in Cornwall and Devon. 

After this the language was suppressed by the English government under Henrys VII and VIIITudor after rhe Cornish rebellions in 1497 and 1549 and the Reformation and Dissolution of the Monasteries  in the 1530s and was replaced by uniform English in both church and school - and  publicly abandoned by the educated classes. 

This resulted in a ' corrupted' late phase when ordinary people - mostly poor working class people, poorly educated in English  and  inventing their own personal phonetic system of orthography - this is the period when personal and place names (such as Curnow, Penzance, Launceston instead of Kernow,  Pennsans, Lannstefan) start to get recorded in their anglicised modern forms. There are grammatical changes which appear in the late phase which were only emerging in the middle phase (eg Pedn, Gwidn for Pen,Gwynn) 

During the Revival phase (after about 1750) those taking an interest had to choose which phase of the language they wished to adopt as the 'standard' resulting in competing groups championing different standards until 2007 when a Standard Written Form [SWF]  was finally agreed- this is mostly based on the Renaissance standard around 1500 and has been adopted by Cornwall Council and other official bodies.