Lack of independent assortment of two genes A and B in fruit fly Drosophila is due to :-

Mendel's law of independent assortment holds only for genes that lie on different chromosomes (or very far apart on the same chromosome).
When two genes lie close together on the same chromosome, they tend to be inherited together. This is called linkage, studied by T. H. Morgan in Drosophila.
Linked genes do not assort independently; the parental combinations appear far more often than the recombinant ones.
Recombination and crossing over actually separate linked genes, and so work against linkage; repulsion is just one arrangement of linked alleles.
So the lack of independent assortment of genes A and B is due to linkage.