She had a good contract with NBC for other primetime movies (according to IMDB after Palace Guard she starred in four NBC movies), so I think it was simply money. I recall when she was later hired at GL, there was press about how much she was paid, and one assumes that SB did not have the budget to make her an offer she couldn't refuse. Or, given all we now know about the mismanagement at SB, perhaps there wasn't enough money to make her want to go back.
I would give them two possible, and logical, excuses for the mess of a story surrounding Eden's exit.
1. As recently discussed, SB had a 50/50 track record of recasting key roles, so they may have felt that by not killing off Eden, there was always a chance that they could recast in the future.
2. It was not unlikely that Marcy's primetime show would fail (which it did), and they may have held out hope that she would return.
Either way, the writing was on the wall for a year that Marcy was leaving. And at the risk of repeating an oft said sentiment, Carrington Garland was well placed to take the thrown as the leading woman of SB, if they hadn't fired her in a lame attempt to get more ratings by hiring a "name" from another show.
Getting back to the article about SB's ratings, it is clear that their constant stress over the failure to attract a larger audience was the major motivation for story and casting, rather than trying to please their already devoted fan bases, which wound up alienating the viewership.