Japan culturally has become extremely modern, especially when it comes to music and electronic tastes. The Japanese enjoy lots of music with synthetic feel and so it works well with Tokyo Disneyland simply because its popular right now. The Japanese audience is partial to this, and so its only granted that its used inside the parks.
While I grant that America is also into synth in music and electronic tastes, it is not as extreme and high end as in Japan, though our pop music influenced theirs, it still took on its own form and shape in this decade. It's something our American audiences aren't very used to. So our taste is more traditional to our pop and synth music.
The difference in cultures also involves a difference in parks, while Japan is adding some more of that modern feel into its parks because it is that popular, the American parks are staying more traditional and only uses modern music in limiting ways suiting our tastes, like Miley Cyrus and the new music from Ms.Menzel. You will not go into new years eve rocking out to Japanese style synth pop, you will rock out to our more traditional styles of music, some with some synth but not used in the same style of the Japanese.
American's just aren't used to the feel of the synth that is used in Dreamlights, the traditional music just feels friendlier to our audiences. Just like how the Dreamlights music must feel friendlier to the Japanese.