I'm going to take a swing at this and say, it's because the costume designer & props made a joint decision, most likely in conjunction with the director, to provide this difference as one of the aids to the audience in more rapidly identifying who was with which unit.
As everyone is wearing approximately the same colour uniform & faces are not easy to identify in the midst of all the similarity, the audience need all the clues they can be given. The helmet choice was quite possibly a part of this too.
99.9% of the target audience would have absolutely no clue as to whether this was completely realistic or not, nor would they really care.
To most of the audience, they may as well all be carrying the same rifle; most people certainly wouldn't know what model it was & would need far larger clues to identify the players.
There would, of course, be some checking with the military advisors, along the lines of "Is it conceivable that this unit could look like this & that unit like that?" If it was potentially possible, then that's a tick in the 'being able to tell them apart' box.
I read somewhere that the Delta guys did have those odd 'skateboard' helmets. I can hear the costume designer whooping from here at that one.
I can imagine conversations along the lines of,
"Ohh, laser sights! Can they all have them?."
"No, for two reasons - one, it's going to start to look like RoboCop meets some kid's computer game… and two, we can't afford to give them to everyone."
"So can't they have those cool telescope sights instead?"
"No, in movies only snipers have telescopes. The regular guys will just have to do without & only the elite guys can have the cool red dot ones."
You have to bear in mind that most of the people who ever see this movie are not even going to be American, let alone experts on their army's military equipment. To the vast majority of the audience, therefore, they're all just "soldiers".
More broadly, of course, this type of detail-skipping happens in all movies; it's only the ones covering a topic you have a specific interest & knowledge in that you will ever notice.
For similar reasons I have great difficulty watching movies about the music business, or those that are actually about movies and movie-making, as they're industries I work or have worked in, so I can spot every glaring error from a mile away. Other people, of course, would never spot them, so simply don't care.