I think aeromech3 mentions good reasons for operating one hydraulic service at a time.
Other reasons might be :
Hydraulic pressure might drop so the services might not reach the intended positions correctly. This might produce a warning or it might not, but it could cause a lock-out - for example of flap or slat WTBs - leading to problems you would prefer not to have.
If, say the gear and flaps were operated together and the aircraft suddenly banked over un-commanded; which service caused the problem, and therefore which will remove the problem ?
Best to operate one service at a time if you can and confirm the correct operation, travel and lock of each position before operating the next. (Apart from normal ailerons, elevators and rudder, obviously, which are designed to be operated together).