For one thing, decriminalization is easy to do. Simply revoke the current criminal laws. The bill would be passed quite quickly. Which would then buy time to figure out a bill for legalization.
Why is this important? Despite popular belief, simple possession laws ARE upheld in Canada, but the people charged are disproportionately minorities and poor/homeless people. IIRC, there were 40 000+ criminal charges for simple possession last year in Canada. That means thousands of citizens who are now burdened with a criminal record, making job searches difficult and border crossings into the US.
So while laws for legalizing pot are getting figured out in Parliament, more people keep getting locked up for simple possession.