Unfortunately, we haven't received a satisfactory answer from the federal government as to why it's taking so long.
As you note, Quebec does the heavy lifting in shaping the eligibility criteria, selecting the candidates and processing them. When they're handed over to IRCC, all that's left to do is the admissibility check, yet completing that component is taking IRCC longer than the time it needs to process other economic class candidates from start to finish.
We don't have a cogent explanation as to why that's the case.