Benedict,

This is a great question. The reason has to do with stress placement. In the word CANADA, the second 'a' does not carry primary stress. Stress is on the first syllable in Canadian English. An unstressed vowel in English turns into an 'uh' sound. So, you get CA-nuh-duh. In contrast, in the word CANADIAN, the second 'a' carries the primary stress in the word, so it is fully pronounced, cuh-NAY-di-un. It is different from the French, but that's only because French is syllable-timed instead of stress-timed and so does not follow the same patterns of vowel reduction. I hope this helped!