They knew that if they didn't get buy-in from the other parties, the next time say the CPC government got in, they were going to go back to FPTP. Now they were very unlikely to get the CPC on board, but having the NDP and Bloc would have made it more politically difficult for the CPC to roll back changes.
The CPC and Bloc were absolutely going to run on any change being a Liberal power grab. Being a minority on the committee helped defuse that argument.
Any sort of referendum was going to fail. It was the CPC's poison pill. All of the parties were running internal polls telling them the same thing.