Hi @Listener001 , I think there are many reasons.
First, derived from what @Rxnin mentioned about the 5 dollar shirt at Walmart, since it is such an iconic and representative band of the 90's and many 90's kids (as me) are now adults with adult money, big stores like Walmart start selling shirts with the logo and the album art on them to appeal to the nostalgia of it. Same reason why so many brands, even luxury brands, have been launching so many anime-related (specially Naruto) and 90's and 2000's cartoon collections.
Related to this, many 90's teens may currently have their own teenagers who are developing their own personalities and taste in music and style, and who may be looking for something more alternative/unique/vintage (at least in their eyes) to feel cool or to try something different. I also consider that Nirvana has became like a staple group for edgy kids, not only on the 90's, so there's that.
Also, just as @AlyssaPandora mentioned, we have to admit that Nirvana's logo and album arts are really cool. For example, I personally love the art of the In Utero album so even if I didn't like their music, I would love to have a shirt with that album art.
Rxnin
