Some artists reserve all their rights and do not allow their song to be re-recorded or printed or published in any form other than the original recording. Each area of publishing is separately negotiable, often with different companies. If a song is not available in tab or sheet music or whatever form you are looking for it's probably because the artist or their representatives have simply not struck a suitable deal in that area yet, or a deal was struck for a limited time and then it expired. That's business.