Why don't they go with Violet-ray instead of blu ray?

[SG 117]

I've known from Science class that different colours have different wavelengths. Red being the longest, and violet being the smallest, with blue being somewhere in between. If the reason for blue lazers is so you can get a more accurate reading due to a shorter wavelength, why not go with violet? They have the shortest wavelength of them all, why not go with that? You could fit even more onto the disks. Is it because they would be too expensive, or because we don't have the technology to create such precision hardware at this time? Or is there some other reason, does anyone know the answer to this?