Did they explicitly tell you that was the reason?
I would imagine for most people it was the change of pace - the fantastic stealth elements and just the general tone of those missions.
EDIT: Sorry dont want to move away from the point of the thread but I just dont think I agree with your assessment of why people liked the Sniping levels so much.
I think that in some ways because most games focus on the player being a full on assualt guy, the things you mentioned hating come part and parcel of that package. I imagine if your a grunt being forced to run at people with guns in the real life military things would be equally messy.
Although I kind of see where you are coming from, ultimately a game where you can be so skilful you never get shot would be interesting - as others have mentioned there are games that try and emulate realism, but I kind of agree with you in that none of them probably emulate what a real life specs op team would have to deal with.
Trust me, that would be one of the most boring games ever. I don't think going further than ArmA makes sense for a milsim.