I'm not sure. Could be cost. It might also be that they need time to test or even redesign to use the new fluids if they have different specs. Another possibility is that no matter what the fluid, if it breaks down or evaporates, the loss of insulation could still lead to a fire by igniting other things, such as the pole or nearby vegetation.

In a case of interesting timing, I heard a transformer blow just last week. There wasn't any fire and the fire department wasn't called. The linesmen showed up a couple hours later to replace it.