Actually they do, and here's why.
For starters, until the Courts shift their mindset to view "Acts of God" (i.e. non man made events) as non reasonable explanations for service delays, the main test for Mandamus is unlikely to be fulfilled in the majority of cases.
Excluding that, the call to close borders to those outside Canada came from the Prime Minister's office, as advised by Health Canada. Following that directive, IRCC determined that they wouldn't be able to land a majority of outland candidates, primarily due to them not being able to physically come into the country.
- A half skilled lawyer would argue that the situation was out of their hands, and thus, the pause on FSW draws, and outland processing. A Court would view that as a reasonable cause for delay as the department couldn't go against the Public Health directive even if they wanted to.
- An even better lawyer would argue that continuing to issue draws for outlanders and processing their applications right up until they got PPR, only to then not issue said PPR, is even more cruel, citing mental health concerns and real life disruptions to lives.