That's because most common is the commonest: https://books.google.com/ngrams/graph?content=most+common%2Ccommonest&year_start=1800&year_end=2008&corpus=15&smoot

That's because most common is the commonest: https://books.google.com/ngrams/graph?content=most+common%2Ccommonest&year_start=1800&year_end=2008&corpus=15&smoothing=3&share=&direct_url=t1%3B%2Cmost%20common%3B%2Cc0%3B.t1%3B%2Ccommonest%3B%2Cc0 https://books.google.com/ngrams/graph?content=most+common%2C...