Although the GI reunion movies had varying levels of quality, none of them came close to matching the appeal of the series. GI wasn't just about seven castaways; it existed in a very specific moment in time that could never be duplicated. Sitcoms in the mid-60s were, for the most part, gaudy, loud, empty and weird. Most shows of that era were honestly atrocious but GI took those same qualities and improbably made them work.
The reunions may have had (more or less) the same format and (mostly) the same cast, but they lacked the sheer kinkiness of the era GI existed. Plus the main thing that worked on the show was the cast and by the time the movies were done, the cast lacked the same vitality; they all had varying levels of a resentment to a show that made them famous but trapped.