Couple of thoughts

One - I don't know. Maybe the shooting schedule didn't allow for the necessary growth? Sometimes projects overlap and you get a kind of a Henry Cavill CGI lip situation.

Two - A big part of the point of the film's setup is the inexperience of the officers involved, particularly as combat is concerned. Not only are they outnumbered by thousands, they also don't have direct experience with combat. Plus, even if they had some combat experience, how much worse would combat be after enduring such a prolonged and harrowing siege?

I’m guessing studios didn't seem to care as much back then, beards weren’t in fashion at the time of filming and possibly not considered marketable for the leading men to sport, I would have loved to have seen more of soldiers look the part, but it’s still a great film, one of my favourites!