Hard to know what they are up to, but my guess is that it probably has to do with rescheduling. There could be a lot of issues there that could lead to "unfairness". For example, imagine a person whose criminality was completed 10 months ago. If the oath is rescheduled, to later than 3 months, then it's back to the seemingly endless "in progress". Maybe in such exceptional cases, the rescheduling could be moved up the queue, or the person might even be allowed, as an exception, to take the oath from outside (unlikely, I think).
Someone suggested that oaths must be taken inside Canada because the PR card gets cut during the ceremony, but one needs to be able to get back into the country. I am not so sure about that one. It would not be a huge deal to simply say "if you are outside the country, then don't cut your PR card, but return it to us when you get back".
I think requiring the oath to be done within the country is simply a matter of some principle, just as the whole idea of the oath itself is a matter of principle. After all sort of paperwork and checks, the oath itself does not make anyone more "qualified" to be a citizen than they were before it. But even in covid times, they will not dispense with it, because of some "idea" that people are reluctant to let go; and they want to keep as many of the in-person rules as possible. Otherwise they could even have 100,000 people doing the oath on video at one go!
