Optoisolator with EL wire won't turn off, TRIAC works fine. Why?
Something is being missed here. The optotriac is quite capable of switching that load current, however the triac in the optoisolator is handling the inverter's high frequency AC differently from your 'triac'. It's designed to control mains frequency AC - 50 or 60Hz, and is too slow to turn off at the very brief zero crossings when it is fed with ~1kHz AC current

Edit: there is a good app note from Vishay here which makes it a bit more clear. You cannot just use the static dv/dt spec - that is what prevents the opto from turning on when it is off, rather you need the (lower) commutating dv/dt, which may not be given.

In fact, perhaps it's not actually a 'triac' but an alternistor that you're using, which would tend to perform better because the semiconductor structure is different. 

The answer in that case is to use a more appropriate switching element, such as a OptoMOS SSR.