It was many years ago, so I don't know if I'll be able to give you a good answer.
There were a number of productive meetings and there was a common goal. Everyone agreed that Canada needed a commissioner to defend the things we cherish. There were a lot of meetings, and a lot of connections were made with communities that may have actual modern treaties. The first nations made all that happen.
Officially, this is a government bill, but it's based on the needs expressed by the main stakeholders, who felt that what it contained was sufficient. When you need a glass of water, you don't want someone to offer you orange juice. When you need water, you need water. That's essentially the message that everyone rallied around.
Basically, for first nations, not getting behind this bill because it has room for improvement could be seen as bad faith or as a way to shirk their obligation to implement treaties.