simcha: because the Torah requires us to wait patiently until Hashem will redeem us and not to rebel against the nations or try to “force the end,” which the Zionist State is doing.
People who support Zionism are implicitly saying that they don’t believe that Hashem will redeem us, because they are trying out to carry out the redemption themselves, prematurely. Not believing that Hashem will redeem us at the time that He deems fit is a violation of the Rambam’s 13 Principles of Faith.
As you mentioned, the original Zionists believed that having a State in Israel was a replacement c”v for Torah, which is avoda zora.