Because that would be illegal:

Federal law — the Hatch Act of 1939, amended by Congress in 1993 — specifically prohibits secretaries of state from attending political conventions, and the State Department's own ethics guidelines also rule out political activity. A senior administration official, speaking on background because the official is not authorized to speak on the record, told CNN, "The law carved out the State Department as having a unique position in the government in that foreign policy, by its nature, must remain nonpartisan/apolitical."

Clinton, who is on an eleven-day trip to Asia, will miss the Democratic convention for the first time in four decades.