Why didn't Durkon Plane Shift to Celestia to talk to Roy?

[Peelee]

I was reading some old strips when I came upon this one and thought, if the evil adventuring party can Plane Shift to Celestia, why couldn't Durkon? We know he has that spell and Roy could have helped him coordinate meeting back up with Haley and Belkar.

Obviously this would have spoiled a major narrative arc, but are there any practical reasons why this would not have worked?

Others gave several excellent reasons, but let me relate it to the real world for a second.

Durkon can't for the same reason that I can list my location as Birmingham, AL, and you can have a car in North America and yet would not be able to just drive on over and have a nice chat with me.

Even ignoring infinite planes, it's a tall order.