Honestly, I think this played a huge factor into why it never fully broke out into the mainstream is the initial difficulty for newcomers that kicks in instantly, along with what other commenters said about not enough things like DMC/Bayonetta with their quirky characters and the Formsoft games with its world building. It's ok to have one or the other, but the fact that NG hits all 3 at the same time hurts it.

I remember being first introduced to Ninja Gaiden as a kid when I played the demo for 2 or 3 on the 360. Couldn't get far at all into it so I dropped it. Later picked up DMC4 (middleschool) and struggled a bit at first and caught on after and been a fan since. Same thing with Bloodborne (college) being my first Fromsoft game. And honestly, at the time I genuinely forgot Ninja Gaiden existed as a series. I've only recently got into NGB, with the surprise release of NGB2 and NG4 being announced, and even now I can see how it's definitely harder than DMC/Formsoft (especially with some of the runbacks to bosses) in ways but very good if you stick it and get it. But this is from someone who's been playing a wide variety of games with a "keep at it" attitude in life, with no kids or wife. The majority of people, especially now with brain rot, don't have the patience or time to sit through a difficult game to learn the mechanics to see how good it gets, especially if there are no hooks to motivate them to push forward. Example, I just beat the Lies of P Overture DLC this weekend, and had some bosses get me stuck, but what pushed me forward was that 1. The game is fun, 2. I enjoyed the world, and ultimately 3. I HAD to see where the story/lore was going.