Because the landing gear (and most other hydraulic consumers) does not have multiple independent hydraulic actuators connected to seperate systems.
It would be stupid to "x-feed" hydraulic pressure and fluid to a system, that may have a leak.
On the 747 one hydraulic system supplies the wing gear, while another supplies body gear and nose gear. However it is not possible to feed among the different systems.
By using the alternate gear extension procedure, hydraulic pressure is locked out. For that reason it is not possible to retract the gear again. On some planes you can revert the alternate gear extension procedure again, after which hydraulic supply is again available. In case of a hydraulic failure, this is however not advisable.
Brgds
SE210