Because it was made to rouse a person in a morning; originally a tune
played to wake the sportsmen, and call them together, the purport of which
was, _The hunt is up!_ which was the subject of hunting ballads also.
This expression is common among the older poets. One Gray, it is said,
grew into good estimation with Henry VIII. and the Duke of Somerset, "for
making certaine merry ballades, whereof one chiefly was, _the hunte is up!
the hunte is up!_" Shakspeare has--
Since arm from arm that voice doth us affray,
Hunting thee hence with _hunts-up_ to the day.
_Romeo and Juliet._