Carmack did execute, they had a working rocket, and with time they could've solved the software issues, but they didn't have the funding to blow up dozens of prototypes.

Carmack drove that project, he wrote code, he built rocket engines himself, he ran missions.

Elon, notably, doesn't do any of that. He just had more money, or was willing to commit more money, to seeing it through. For Carmack it was more of a fun diversion (the X Prize) than a business he wanted to build.