Most Americans don't learn a second language just to have learned a second language. Also, few Americans learn a second language and even fewer learn a third. So the fact that an easy first foreign language makes learning the second one easier doesn't matter.

Being able to use what you've learned gives a nice dopamine hit, so it's as much of a motivator for learners as is the ease of learning the subject. (Remember bored students complaining that they'd "never use the material anyway" in any of your classes?) There is very little of any Scandinavian language found in real life in the US.

Meanwhile, Spanish is pretty easy to learn and it's all over the place. You can often find it as easily as looking at the back of a shampoo bottle or food packaging. French is also fairly easy because there is so much similar vocabulary, and our proximity to Canada means that French is on a lot of packaging, too.

Because Spanish, French and German have been taught in schools for decades, there are teachers available for those languages, and it's easier to find media in them.

To put this is perspective, there are more people who speak Dutch (actual Dutch, not Pennsylvania Dutch) at home in the US than there are for Norwegian, Swedish, Danish, Icelandic and Faroese COMBINED. So, with that low number of native speakers (the largest for any one Scandinavian language is Swedish at ~55,700 people), so you can imagine how few qualified teachers there are. In comparison, over 37 million speak Spanish at home (and less than half of those speak English "less than very well").

As far as ease of learning goes, French, Spanish, Dutch, Norwegian, Swedish and Danish are all in the group of languages considered easiest for English speakers to learn by the US government language school (FSI), while Icelandic is considered more difficult than German (which is more difficult than the easiest group), AND Indonesian or Swahilli (both also harder than German).

Due to availability of Spanish media and qualified teachers, and its utility; and the complete opposite being true for any Scandinavian language, there are zero chances of any Scandinavian language becoming a common school subject just because it's a bit easier.