LOL, "sucks" may have been a bit of an exaggeration. Some are really good. I guess the better way to say it would be to say that they're all rather similar. If you've seen one, you pretty much know what a lot of the rest of them are like. And they're alike in a way that becomes rather tedious if you watch more than a few. "Life sucks, war sucks, blah blah blah." Compare to, say, Pearl Harbor, Saving Private Ryan, Raiders of the Lost Ark (as much a WWII movie as Gump was a Nam movie), and the Great Escape. All quite different. Or South Pacific, LOL. You might say Vietnam was too gruesome for a musical, but we dropped a damn atomic bomb on Japan.

A follow-up question, not necessarily directed at you: Why aren't there any WWI films? I can only think of one, Lawrence of Arabia. That war produced really great literature, but not a notable film in fifty years. All the things that make Vietnam a good setting for a movie (moral ambiguity, fear, atrocity) are even more present in WWI.