It's honestly always been niche and never mainstream. However, it was less niche compared to now back on the Xbox with Ninja Gaiden 1 up until about Ninja Gaiden 2 or so. Ninja Gaiden 3 kinda decimated Ninja Gaiden's reputation.
Well, it’s just hard to find people that talk about it. And for all the reviews (which take a while for me to find) I have seen praising Ninja Gaiden 1 and all as well as being said as one of the best hack and slash games of all time, I noticed that it is usually only pretty much scratches the surface level of what made the game great or when just comes to general trivia of the game's lore...
there is always some misinformation here and there.
Majority of the strangers that have played Ninja Gaiden 1 on the internet I bump into pretty much only played through Ninja Dog/Normal difficulty once and then just stopped playing despite them saying that they liked the game...
which I found a total shame because they are missing majority of the content that made NG awesome (Like the new varied enemy designs, learning curve, and the progression).