Using the correct file to sharpen card scrapers is quicker because it requires less work to remove the scratches from the file.
Single cut files leave a smoother surface than a comparable double cut file.
Single cut files are also known as farmers files and are suitable for sharpening agricultural and garden tools.
I had thought that bastard cut mill files were coarse but, the example shown has 45 tpi and leaves a very smooth cut.
The right tool for the job.