Because currently the roses breeders have not been able to produce a pure blue rose, its the holy grail of the rose world.
no? Interesting. Is there a reason for this?
Roses are not able to produce the blue colour naturally, it is about plant geno code, every much like humans DNA, it means cracking the code, and producing hybrids. The blue moon rose is a hyrid tea rose, the black beauty and David Austin Roses are what as classed as old world roses. Genos that have never be crossed to produce a flower that is man made.
Therefore a stronger stock base. The hybrids now are more factory looking, the roses you buy at a flourist, many are produced with scent ( My school of thought is a destruction of G-ds creation a rose without scent
).
So the Blue rose is solely a creation of man, given the challenge over hundreds of years, and yet none have managed a true blue.
You see Roses are one of the oldest plants on this planet, and G-d never saw fit to include Blue in its colours. Man basically wishes to out do G-d himself.
But its not really my field I only have such a small knowledge base on horticulture.