Why does cross-pollination produce more genetic variation than n the offspring that self-pollination produces?

Self pollination occurs in plants with both male and female flowers. The plant can pollinate itself, but only with the genes it already has. Cross pollinating with another plant (and not even necessarily of the same species!) means that another plants genes can make new combination.

A plant with genes dominant only (XX) only makes dominant (XX). Crossed with itself, that's all you'll ever get. Add in another set of genes and you could get XX, Xx, or xx! And that's for just one set of traits. You could get XxYY, XxYy, XXyy...Overall, self pollination reduces variation by limiting the available pool of allele types to work with.

While self-pollination happens when a plant fertilizes its own eggs, producing offspring with genetic material identical to or very similar to the parent plant, cross-pollination involves the transfer of pollen from one plant to another, often between different individuals of the same species. This process results in offspring with genetic material from both parent plants, leading to increased genetic variation.