Because they are afraid a new republic would not honour agreements made by the Crown.
I'd be afraid too but that said these are increasingly powerful times for native people. The amount of 1st Nation's opportunity I see on the coast where I live especially in commercial fishing right now is staggering. All that really stands in the way of it is the capacity but there's a few very obviously very successful young fishermen that are attracting notice of their peers and a path to real success. It was a snap for DFO to sink most of the non-native salmon industry when caving to America during the Pacific Salmon War but if push comes to shove again caving to the US will be whole different kettle of fish when it comes to keeping native fishermen off the water. Ironically it'll probably be the Crown turning to the Monarchy for help.