[lil_biker]

Why doesn't rain on the superstructure at an electric substation conduct electricity back to the frame?

[Credendovidis]

Why doesn't rain on the superstructure at an electric substation conduct electricity back to the frame?

Who says it does not?
Go there on a rainy day and see for yourself : frequent sparking does occur.
Note that pure water is a bad conductor. Water needs to be polluted to conduct electricity.
For instance with salt added to the water it becomes an excellent conductor.
Rain is always polluted by microscopic `seeds´ of stone or sand, that allows water vapor to collect and build up around the `seed´. Depending on what `seed´ was used to form the raindrops, conduction is worse or better.

Piped water is on the contrary loaded with salts and other pollution. Making it often a better conductor than rain (with exception of `acid rain´).
;)

[Handyman2007]

If you could ever get to see one of the substations up close , you would see so many ground circuits it would boggle your mind. THat is also why lightening is not a great concern to these facilities, They are VERY WELL grounded to delute the effects of lightening and arcing.