The earliest recorded reference to 'mike' as an abbreviation for 'microphone' appeared in an edition of the 'Melody Maker' in 1927. The first time 'mic' appeared in print was in A Berkman's 'Singers' Glossary', published in 1961. The 'when' part of your query is, therefore, 'at some time between 1927 and 1961'...I'd guess in the Fifties. The 'why' is probably just because it's shorter and the pronunciation is 'mike' in both cases, as 5029 says.
Thanks, both of you... I asked because I'd never come across mic until about 5 years ago; yet it now seems the more common form in print. Just one of those unpredictable trends, I suppose.
Believe me, Rojash, you could fill hangars with stuff I don't know! As I've explained on AB before, I have excellent reference sources and - being retired - ample time to make use of them. Nevertheless, my thanks for your kind comment. (I like your 'Coke' response above...nice one!)