Because TRVs still need to go through background checks, just like any other application. This means that IRCC is working with the applicable agencies across the world to complete this in order to determine whether you are who you say you are, did not commit crimes in other countries, are not affiliated with any gangs, and are in general eligible to enter Canada.
Processing time depends on your background and travel history.
Currently with covid, you can expect that all agencies are understaffed and backlogged.