See my post #33 for an actual list of senators who voted against excluding this element last time it was put to an up and down vote. Only Kyl's vote was a parliamentary maneuver to allow him to bring it back for later consideration.
The remainder either think it is a good idea to include gang bangers and criminals in on amnesty or are simply too afraid to stand up against the race hustlers at La Raza, MALDEF and Mecha on what should be a no brainer.
If there is any explanation other than these two, I have yet to hear it.
It's possible the impression was the amendment was too far reaching, it did include some considerably lessor crimes.