If I search for Canon raw on Google the Canon owned websites that I see writes it all uppercase; RAW.
One of their pages that I find even makes note of that:
> The letters RAW do not stand for anything – it's just a convention that RAW is usually written in capital letters – and the names of RAW files from Canon cameras do not end in .RAW.
Perhaps the combination of that and the old .raw filename extensions on old filesystem implementations where everything appears uppercase (since camera firmware is slower to catch up, this persisted for years even though contemporary OS already had no such limitation) made it stick.