It was released in 2016, only two years after Atom and Slack and a year after VSCode. Not sure I'd call that late.
By that point Electron had gained too much momentum and public mindshare to compete with without a big marketing push and flagship project at least as significant as Atom and VSCode were. Positron never got either.
Electron apps were popular, not Electron itself. Even now there are only 4-5 dozen popular apps using it, while I probably run across that number of Qt/GTK apps a month. It's a sort of weird dichotomy it finds itself it.