Because a current is only induced in a conductor by a changing magnetic field. Only a changing current can produce a changing magnetic field (in stationary conductors). Now, technically, a DC current _can_ create a changing magnetic field if that DC current is changing in _magnitude_ but not _polarity_--e.g. from a positive current to a more, or less, positive current OR from a negative current to a more, or less, negative current (if you consider "AC" to mean a current that "Alternates" between positive and negative).
http://en.wikipedia.org/wiki/Lenz's_law
