Why did Egyptians forsake the gods of their ancestors?

because they were pagans, and like most pagans worldwide unfortunately their minds gave in easily to the intimidation and fear enforced by monotheistic organized religion. Like most pagan cultures, though to some extent outdated today (who would seriously believe in the existence of obviously fictional characters?), their culture was incredibly complex and its view of the cosmos and afterlife very deep...so it is such a shame that they gave in to Islamic imperialism...(but they notably where Christianized before it, i think, not that it makes much difference - from one organized religion to another...)