Russia and Ukraine

[Thumpalumpacus]

Would have had to no matter what the circumstances were.   They're not fast, they're not stealth, they're not even quiet.

At 150' AGL, radar is not going to matter until they're very close to the radar-site, meaning tracking time, or even response time, is going to be foreshortened. Detection at a mile when the chopper is doing 120 IAS gives the defenses about 30 seconds' response time before the attack commences

And while the choppers were certainly heard, at night, with transponders off, there could well have been enough uncertainty about ID/IFF that any SAM assets in the area withheld fire until confirming ID.

I'm just throwing out a couple of possibilities to explain the lack of return fire.