He said in one of the uploads that he would fall on his sword if the power alliance got rumbled. On initial upload we didn't see Chris or Mark say anything, but on highlights they all did volunteer, all 3 of them. However I think if Chris has been more adamant and insisted CHOOSE ME and stepped forward, then a) he would have got a lot of respect from the viewers for staying true to his word and falling on his sword to protect Ashleigh b) he would have had the opportunity to slay Steven himself, his arch nemesis
He must know that there would be ZERO chance of him being evicted against Steven.