It is apparently obvious that you were too young to know how popular an action star, Arnold was in the 1980's. After making his breakthrough as an action hero in the films "Conan the Barbarian" and "Conan the Destroyer" he had a run of commercially successful films: "Commando" (1985), "Raw Deal" (1986), "The Running Man" (1987), "Red Heat" (1988), "Predator" (1987), and "Total Recall" (1990). I think that after doing "Predator" there were talks of Schwarzenegger and Bruce Willis being involved with a "Seargent Rock" film, but the project never came to fruition according to
a Gavin Ford piece in the "Star Gazette". I think this was about the time that the WB was working on a project called "Batman", and I have a feeling that they wanted to establish that franchise first before taking on one with a so called non flagship character like Sgt. Rock. The project did resurface again in 2002 after an observed resurgence of war films (post 9/11), but I am sure it died this time because of the different opitions that Arnold had at the time (he wound up doing "Collateral Damage" and "Terminator 3") and the fact that he eventually ran for Governor of "Culifornia".