Perhaps they don’t really want a regime change, or at least not an orderly, prosperous one. Perhaps getting the Islamic Republic to dig its heels would prevent improvement of relationships with the world, lifting of sanctions and economic growth that would create more resources for military strengthening. Perhaps having the ambiguous big bad Muslims in power makes it easy to manufacture consent for arbitrary bombings, both domestically in Israel and abroad. Perhaps keeping this enemy keeps people in Israel motivated to comply with the right wing policies of some fractions.