Presumably because Bitcoin’s privacy sucks and if they were selling lots of bitcoin and depositing it their banks would have questions?
"Describing the process, he said the gangs would convert their dirty cash to bitcoin, then used the cryptocurrency to pay people who sold fake streams on Spotify, which is a Swedish company."
https://www.thelocal.se/20230906/how-swedish-criminal-gangs-allegedly-launder-money-through-spotify
I want to know more about how these gangs converted their dirty cash to bitcoin in Sweden, and why the Spotify step was needed at all. Seems like a cumbersome mechanism.
Discussion
I think you're right, they thought they could explain the Spotify income as legit earnings from their gangster rap, but not the Bitcoin.
Financial privacy doesn't exist at banks so you need to have a good story about what you earn.