The real gene-pool for Jews is of darker hair and eyes. Blond entered the Ashkenazic pool because of years of persecution and attacks (rape during pogroms) in Europe. The Jews in the Sephardic communities may have had the same experiences, but the Arabs had the same dark hair/eyes as the Jews, so there were essentially no differences in the makeup of the genes.
There are definitely more Jews of Sephardic lineage in E”Y than Ashkenazic. But I am not sure that the number of sheital-wearing Sephardim contribute to the difference you are seeing demographically (I think Sephardim tend to use scarves and kerchiefs more than wigs).
There may be an economic factor as well. Darker hair is easier to come by, and therefore darker wigs are cheaper than blond ones (especially in the custom/semi-custom market). In an economy like Israel, it may be that those women who are natually a darker blond will opt for a cheaper brownish wig.