The voltage at which electrons are released at the negative junction and the breakdown voltage both appear to be lower for materials of higher dielectric constant. (20721) Since hard glasses generally have lower dielectric constant, quartz was selected over a glazed ceramic as the insulator material.
Voltage grading provides a uniform electric field along the insulator, thereby preventing a localized accumulation of charge and consequent flashover. (22)
I wonder if a "leaky insulator" would help prevent too much charge build up.
If you have a resistance of 100 MegOhm/square the 2 MV stand off might be
set up to leak .1 to 1 % of the total current. That would help keep the field
uniform over the whole insulator.