Is there a reason why Istishia is not on the druid diety list?

First line in Istishian dogma Everything is interconnected and cyclical.

I mean, if Kossuth has druids there is no possible argument that Istishia doesn't in my mind. You could probably also make a case for Akadi. I won't get into Grumbar though - that's been argued enough now.

I don't think any of them should have druids, personally!

as has been said plenty of times i would guess, none of the elemental deities are considered nature deities..

The only reason Kossuth is often an exception is because his list of worshippers specifically mentions druids, even though he is not in the list of nature deities on page 90 of the Faiths and Pantheons sourcebook.