I think it's because they are trying to stay traditional, the same reason they won't bring in Hawk-eye.
I think the biggest reason has been that they haven't felt need for lights. Sun sets in Paris at 9:45pm which is later than at any other slam. In London it's 9pm, in NYC it's 7:30pm, in Melbourne it's 8:30pm. I think they thought 9:45pm is enough late to stap play.
But now even 250s have lights and so should RG have too. But I think they aren't too willing to make any investments to RG before they know a slam can be held there also in the future.
And I don't think traditionalism is the reason for no Hawkeye, it's used nowhere on clay. Hawkeye just has some issues on clay.