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Because that is the standard rule in English. The OED says:
Hence, in modern English, C has
(1) the ‘hard’ sound [k] before a, o, u, before a consonant (except h), and when final, as in cab, cot, cut, claw, crow, acme, cycle, sac, tic, epic;
(2) before e, i, y, it has the ‘soft’ ...