A possible reason is the restriction in Australia that makes hiring non-Australian actors (or crew) difficult. In the"making-of" video on the original Thorn Birds DVD, the director states that they investigated filming it in Australia but were told they could use only a few (I forget how many) American actors and they had to include more Australians.
This was apparently filmed on location in Australia and a lot of the minor parts were played by Australians so that might have been a factor.

No she continued to act although as of now it looks like she hasn't done anything for a few years.
It could be that neither her or Byan wanted to do it and maybe she wouldn't do it without him.

I think she retired from acting after she married Bryan Brown.

Why didn't she return? She probably read the script!
Have a good time, all of the time
Viv Savage - Spinal Tap

I don't know for sure, but I think she should have. It doesn't matter if they changed all the cast but they should have had the two leading characters Richard and Rachel for it to be believable/successful