Main reasons :
The 5970’s case design is a masterpiece and unique only to the 5970. I don’t think it has been reused in other references. The closest would be the 5270, but the lugs and case band differ significantly.
The 40x13mm is perfectly proportioned. Also, 40mm is just a number, the case design makes the 5970 wear closer to a 38mm. Sometimes I feel it’s about the same size as my 5153.
I prefer a chrono with a tachymeter. Gives it a sportier look.
Some pics :