I guess they thought that inheriting 25 years of C code was more trouble than designing a new editor from scratch. But you'd have to ask the devs why they decided to go down that route
Rust + EDITOR.COM is kind of like remaking/remastering an old video game.
CUA-style menubars aren't that uncommon in textmode editors. Midnight Commander's editor has traditional menubars with much more extensive functionality, as does jedsoft.org's Jed editor. Both of these also support mouse input on the TTY console via GPM, not just within a graphical terminal.
Look at the amount of contributors here. This project was probably some strategic investment. It did not come to existence overnight.