That's an interesting Q, as there's no single answer or reason behind this phenomena. CBSA generally treats China packet truckloads as High Security risk requiring full inspection due to certain drugs shipped this way, with some recent improvements attributed to changing China laws limiting drugs export. They also use packet truck loads slowdown as a safe way to press Government for more funds and staffing quotas because there're no financial penalties for failure to deliver them on time. CP has a set of own priorities behind slowing down packets processing. It seeks to ensure maxed profit with minimal workforce, which is unionized and constantly demands higher pay, pensions, and decreased workload. CP also seeks to continuously increase share of revenue spend on executives compensation, and the packets don't notably contribute to it. CP leadership seems to keep hanging on to previous tactics to compromise CP as a viable public service until changes in power would allow to proceed with previous plans towards CP privatization.
Retail sector of Canadian economy is also interested in China packets delay, including giants like Amazon with very deep pockets for lobbying. Large NA couriers are also lobbing China packets slowdown, since they are out of the loop in that shipping segment, while it lowers their shipments volume from US to Canada. Canada Parliament Report on CP future didn't even mention the packets delay dilemma, sending all the wrong signals to CP and CBSA execs, possibly driven by lack of proper due diligence, retail and courier lobbies, and lack of public and MPs pressure elected in low income districts. Canada Ministry of Public Services appears to set low to no priority and having no publicly discussed action plan to resolve the systemic packets delay from Asia. Objectively, packets are used primarily to import low cost small electrical and electronics items, some clothing, small tools and items for home that are not manufactured in Canada anyway, and do not represent any threat to Canada retail or manufacturing sector, but contribute largely to support low income population segment who's civil rights are deprived by Canada Post packets delays.