Why do offspring produced by asexual reproduction show no variation from the parent?

Because it has the same exact genes as the parent. Unless a mutation is shown. The two daughter cells are identical to the parent because there is no other second parent to cross genes with

Explanation:

genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring).