Because being 'pro-Israel,' or Zionism, is an ethno-separatist agenda, ie that Jews should live in their own homeland separate from other cultures, and the Pittsburgh synagogue shooter was also an ethno-separatist, a white nationalist, who believed that America is for white Christians only, and thus Jews in America are an 'infestation.' Not much different from how Zionists view Palestinians really.
Liberalism OTOH calls for diversity, that people from all cultures, backgrounds, and religions can live together in peace and equality while maintaining their various ethnic identies, a concept which conservativism (which requires conformity and assimilation) rejects outright.
And let's not forget that the President called white nationalists 'very fine people.' But of course, you already did.