In a word, 'demand', for the various reasons that have already been identified. I should imagine it is not the most straightforward of places to organise ground handling as well.

When I flew there with VS, there was no duty free on board. The crew explained that that was because it was at risk of being stolen on the ground in HAV. The plane also had security around it while it was on the ground. Theft of parts was also not unheard of.

I think BA last used to fly there in the 90s. Fascinating place.