It would be great if an expert could share some information about this, but in the meantime here is my best guess, based on googling. According to the following link, e-file to CRA opens Monday February 22nd.
I note that no brand of tax software is CRA approved for 2020 yet, which also suggests the delay may be at CRA.
We are happy to report that the T777S/T777 forms have been updated and you can now complete entering your employment expenses. We have included below our TurboTax FAQ link to help you with this if needed.
Thank you for your patience.