The idea of learning a language through songs is not new. People have been recommending it for as long as there have been records.
What has not existed until recently is a language course that is actually built out of songs — not songs borrowed from the charts and analysed afterwards, but songs written backwards from a frequency list.
Why Nobody Did This Before
Consider what it requires. You need reliable frequency data for spoken Spanish, fine-grained enough to order a hundred lessons by it. You need each song engineered around a small set of chunks that recombine. You need every one written, arranged, performed, recorded and produced, with a lyric video for each.
Any one of those is achievable alone. Together they were, until fairly recently, a linguistics department and a record label working to the same brief for several years.
The economics simply did not allow it. That is the honest reason it did not exist, and it is a better reason than anybody being first to the idea.
What Changed
Corpus data became available and detailed. Production costs collapsed. Making a hundred professionally produced tracks stopped being a capital project and became a long piece of work.
The idea was always sound. The build became affordable.
Why It Matters What Order They Are In
A playlist of Spanish songs is pleasant and does very little, because the charts are not ordered by usefulness. You get a beautiful metaphor about the sea in track one and the word for "want" somewhere in the eighties.
Ordering by frequency inverts that. The most-used phrases arrive first, so the early hours do the most work — which is only possible if you built the songs yourself.
About Outputly
Outputly's songs were written to a frequency list rather than chosen from the charts, which is why the first album covers so much ground and why this could not have been made a decade ago.