I'm not surprised, to be honest. The first game was a success largely because of the interesting hook, which is that Senua is schizophrenic and they immerse you inside her mind by having voice constantly whispering in your ears, or sometimes a voice will rise above the murmur with something helpful, or not.
The actual game play though is not very good, it's actually pretty boring. So even if you were interested in that hook you probably played the first one but you're over it, and if you weren't intrigued enough to play the first one then you'll probably miss the second one too.
I think there are a multitude of reasons for why it may not be doing well, but personally I had no clue it was coming out this month, I don't trust my PC to be able to run it (rtx 3070 isn't enough for a lot of high fidelity games these days), and I got hellblade 1 at full price and beat it in under 6 hours. I'm looking forward to playing it at a lower price and on better hardware in 4 years or something.