I think the reasons are probably three fold.
First, they had enough to do in the last minute just getting the convention together. Arranging for secure submeetings to hammer out an platform is going to make things even more chaotic\
Second, arguing over a platform is going to expose fissures in the party that they want to keep covered. Is rescuing the kids from the Clinton’s sex farm part of their platform? What about addressing the systemic racism against whites? How do you thread the needle of encouraging Republicans to vote by mail, while at the same time establishing that voting by mail is fraudulent? Are masks a good way to get the country back to working and save the election for Trump, or are they a symbol of our slide into a communist hellscape?
Third, its difficult to create a platform when decisions and policies are made on a whim and can be changed a few days later.