Because "zoo" is an abbreviation of "zoological gardens". Such abbreviations often change their pronunciation to conform to monosyllabic pronunciation rules, which can sometimes be different when not followed by other vocalic sounds.
Thank you Mark. I should have realised that seeing as I know zoos are short for zoological gardens. What is the meaning of 'zo'. I am too lazy to look it up - and besides you are more informative than the internet ;o)