Why is Red hair dye so elusive for staying on hair?

I’ve been dying my hair red for a couple years and it lasts now because ive been doing it so long. The thing is red dye always has a hard time lasting. I hear this is because the molecule size is too large , BUT I think that’s only regarding the size of the molecules on natural red hair and not dyed hair. With nano technology and improved dye colors and cosmetic formulas why haven’ t we found a way to remedy this problem in red hair dye?