Out of curiosity, why don't you run red?
Stamina, mostly. I've never really been truly fit. After about 6 km I kind of hit the wall. The occasional longer race is ok (Stumble, Ultralong greens, and I've done a few 10K trail runs) and I will do red on physically easy maps or sprints, but I couldn't do that every week. And I slog/drag through the end of those longer races. I also have wonky knee to think about - having just increased training hurts it, and I'm not even really doing that much more (only running 1X extra per week right now). After a particularly long course or physical terrain I often come back lame.
I did try moving up, probably about ten years ago now, but my technical skills weren't that good then and I went OT or DNF probably more than half the time on red.
So I'm in my comfort zone, and I only have about 1.5 competition years left anyway before I age into green, so I'm just waiting for that rather than try to move up now.
Makes sense - I didn't know about your knee!!