Curious as to why anti-egfr drugs have been largely unsuccessful for ovarian cancer treatment?

Clever cancer: Cancers are composed of populations of cells which divide and multiply rapidly and which have various abnormal metabolic pathways. Anti-egfr drugs are designed to block one important pathway whereby cancer cells promote blood vessel growth to feed them, but they can activate other pathways to do this when the drug is used, or they can mutate so that the receptor isn't blocked by the drug.