DMR are long, and most players fail to operate them correctly. often players are sticking the barrel outside of cover giving away their positions, and or perched on a window with the barrel sticking several inches out of the building, not only giving away their positions but also their line of sight. The other issue, you can't snap shot with them when in cover and you typically have to change to a sidearm or close quarters weapon for closer engagements.
optics are all preference and mostly cosmetics in my opinion. At most all you need is a red dot / eotech / reflex sight. The thing with optics, they do not pickup where the bb's drop and due to the sight over reaching the effect bb range players typical forget this and think because they see a target they can hit said target.
m16's , ak's , m4 ' and g36 are the most popular because they are the best mid range weapon systems, they can be used in cqb and used at distances add in a hop up and tight bore barrel and they shoot lasers. Thus why you see everyone rocking the same weapons type.
I would suggest just upgrading your scar l add in a hop up and a tight bore barrel. The difference will be night and day.