Because of an inability to implement this in a meaningless primary that probably had a tiny fraction of the participation that November's election will have.
A mail-in general election will be an even worse fiasco. The lawsuits and claims of fraud / voter suppression are going to be a gigantic smokescreen hanging over the results for months. All the usual horseshit you see with keeping polls open late in cities (while the Dems try to roust people to go vote) will be magnified with claims of mail trucks disappearing in North Philly and calls to honor all ballots mailed before December 1, yada yada. This is going to be an exceptionally ugly experience for everyone.
It should be avoided at all costs.