perfidy

The ugly irony is that the "U" in "UH-60" stands for "utility"!

The short answer is: I don't know why they appear so vulnerable. Maybe it's an appearance, given a whole lot of all sort of helos are flying over there at any given moment, and they don't get hurt.

The long answer is:
I don't know why small-arms would bring down a Blackhawk; they are supposedly armored to withstand 23mm hits (http://www.fas.org/man/dod-101/sys/ac/uh-60.htm). Maybe that armor covers certain portions of the aircraft, such that other systems might be more vulnerable than others. That might make the difference between outright destruction and being damaged but able to limp back to your base. "Black Hawk Down" talks about UH60s being taken outright by RPGs abut also damaged by small arms which were themselves non-critical hits, but the damage affected other systems which together, over a few minutes, caused crippling damage.

So (arguably) modern anti-aircraft artillery (AAA) systems like ZSU-23/4 and 2S6, say, would certainly do the job, but I doubt such systems are operating in Iraq. AAA/ADA platforms are associated with specific radar systems that render them readily identifiable, and therefore readily blow-up-able by remote (TLAM, say) or by manned SEAD aircraft.

RPGs can do the job as well, and they are not confused by chaff or IR maskers due to the simplicity of the munition. Thing is, you have to be really close and really lucky, admitedly, to hit a flying moving target with a RPG. Small arms fire is similarly limited.

So what's to be done? For starters, I agree with your assessment of night ops. Maybe if you have to use helos to get your people there, do it at night. Night ops can complicate every aspect of the mission for obvious reasons, but US forces are skilled at them (some units more than others of course).

Otherwise, at the very least, fly higher. Helicopters are most vulnerable when they are disgorging their infantry, picking up wounded, etc. RPGs and small arms aren't much use beyond 300m. Drop down fast and hard; make that infantry fast rope while the bird's moving!

Go in harder. Pair transport birds with either "gunships", ie Blackhawks fitted out w/ rockets and machine guns to taste, or bona fide attack helos like AH64 or Marine AH1 (I don't think the Army has any more of the latter). I doubt bad guys will flip an RPG at a UH60 if there are attack helos about. And there aren't any tanks to blow up, so these attack helos have nothing better to do with their time.

Also, coordinate land and air attacks. If the bad guys hear rotors and are looking up, say at the northern sky, plan to assault on the ground from the south. This works especially well in urban areas where the rotor noise can reverberate amongst the buildings and appear to come from several directions at once- some pilots train to do this. Keep them guessing about where you're going until the last possible second.

But of course, if I thought of it, the people who do this stuff every day certainly did and it doesn't appear to be as effective a tactic as they might wish.