I agree with some of the answers here - up to a point.

A lot of mechanical delays are caused by problems that do not threaten the safety of the aircraft inflight but which nonetheless are required to be fixed according to the airline's own operating procedures. The engineers on the ground do a great job but they cannot be everywhere at once and sometimes mechanical delays are caused only by a failure of operations staff to work round the problem more sensibly; on a high-density shuttle route, 3 hours does seem a long time to be held up.

Alastair