1) Because much of the other major and minor parties specifically target the Chareidi community for harm in various manners. Therefore the Chareidi community needs advocates to counter these anti-Chareidi efforts. Additionally, it isn’t true the other parties run to govern for the benefit of all Israelis. Meretz’s goal isn’t that nor is Yesh Atid’s.
2) As a general by and large rule, native Israeli Chareidim are more interested in Torah than in economic success. They’d rather be an impoverished talmid chochom than a rich baal haboss. Their goal in their lives are to be full time ovdei Hashem rather than a middle class lifestyle.