Utility Donations and the Forby Amendment
Certainly Com Ed and Ameren have a lot at stake in the vote. And they've given a lot in campaign contributions to incumbent senators. Much of the money they gave would be illegal in most other states, either because it came directly from the corporation (most states ban direct corporate giving) or because of the size of the donation (most states limit giving from each donor). Certainly, neither Com Ed or Ameren could have given as much to leadership as they did, if Illinois had laws like most other states do.
Some Senators may have cast their vote because they think it was the right thing to do. But some other factors may also have been at play, to varying degrees with different Senators. Campaign giving by utilities and transfers from leadership PACs affect all of them.
The attached spreadsheet lists each Senator, how they voted on the Forby amendment, if they were targeted in 2006, when their seat is next on the ballot, and how much they received in 2005-2006 from electric companies (and, if 2006 targets, from leadership.). No one explanation likely covers all 59 Senators. But the answers to most of the votes can likely be found in these factors.
Forby Amendment Analysis