I was looking at that last night. It's possible that they were turned off or the crew wasn't watching them. Maybe they got accustomed to thinking they couldn't get hit and let their guard down. I'm surprised that it sounds as if the Russian navy is giving up on the ship so easilly.
While I don't think this is unique to Russian vessels, there has been talk of the fact that one of the search radars on the Moskva was only capable of a 180 degree search, and the Bayraktar that Ukraine had put up near the ship was doing a good job of having them look that way, while the missiles approached from the other direction.
If true, it's another example of the way in which Ukraine is clearly using creative and innovative strategies to launch attacks which are leaving the much more top-led Russian military floundering, as they struggle to adapt prevailing doctrine to an adaptable foe. Also worth remembering that quite a lot of the Ukrainian high command will be only too aware of Russian (and probably Soviet) military doctrine, so will have a pretty good idea where the holes might be.
The weather may also have been a factor - there were some filthy storms in the Black Sea at the time, which would have made it a lot harder to spot low-level (I think the term is "sea skimming") missiles amid the clutter of a very churned-up sea.
Whatever, this is definitely a massive morale booster for the Ukrainian military, and - presuming anyone gets to hear about it - a bit of a morale-sapper for the Russians.
It may be that they got very lucky with this, but it does open up the possibility of attacks being made on other Russian ships in the region, which is bound to be concentrating a few Russian minds along the way.
Also, it seems likely that the absence (apparent) of these missiles in the first 5 weeks of the conflict may have led Russia to assume they were non-operational, or incapable of attacking their ships. Complacency could well have been a factor. I guess that will be less the case from now on in...