We need a Citizens Assemble partly because the existing system doesn't work; despite the science and knowledge the government has been going backwards on strategies that impact climate change.
Also because parliament is full of lobbyists who are there full time influencing MPs in favour of policies that disregard climate change.
And because MPs rarely if ever canvassed or were elected on climate change, therefore they have little mandate, and a reluctance to do anything when it contradicts what they were elected for.
It also enables parliamentarians to continue with their other work while a new body gets on with the job of trying to prevent climate breakdown.
And also quite possibly because of a level of corruption or at least dishonesty that leads MPs to make decisions based on their own benefit.
Citizens Assemblies can bypass all of them very simply. The processes for selection are not fixed but really simple to implement, and not expensive. The CA process is also very familiar, we use it all the time in the justice system, so we are pretty comfortable with it.