Why is it not allowed to dispatch an Airbus A320 with an inoperative ELAC 2?
As pointed out by this discussion on PPRune, ELAC 1 only controls elevator actuators hooked in to the Blue hydraulic system. ELAC 2 is split across Green and Yellow.

If ELAC 1 is the only operational computer and the aircraft enters emergency electrical configuration, blue hydraulics will be lost until the RAT deploys. Further, the fallback for ELAC 2 (G/Y hyd) is SEC 2, which is not powered in EEC. Thus, the aircraft will have no primary pitch control.

The solution is that if the fault is in the ELAC computers themselves, to swap the units before dispatch.

In general, if there's something weirdly asymmetric about the A320, the first guess should be something to do with EEC. This is because EEC, by design, tries to only power one unit, typically number 1, of each system.