Because the still-aloof and faithless existence Jєωιѕн Nation - which refused to carry out the very mission for which it was created - is objectively contrary to the Divine Plan, for which Our Lord Jesus Christ died on the Cross.
So you actually dispute Israel's right to exist. I agree that this was the position of Pope St. Pius X, and even surprisingly that of his less than orthodox successor. But The Church was less vocal about this 'continuing judgment' following the secrets uncovered under former nαzι rule. I don't believe that Pius XII lobbied against the UN partition. But correct me if I'm wrong. And in any case, I don't think you'd argue this is a matter of dogma, or even something somewhat less, that a Catholic is in any case bound to oppose the 'zionism' that consisted of creating an independent Jєωιѕн state (again, whatever Jєωιѕн actually meant, in fact, when compared with antiquity).
I don't defend Begin. I don't defend the King David. I don't defend ginning up Final Solution death tolls to the point of exaggeration - because that's fuel for the 'deniers', now, at this point. I don't defend the Lubavitchers or their schemes to get the Jews for Jesus should the 3rd Temple be built and a new Sanhedrin created to pass such a death sentence. I don't defend what Netanyahu and the orthodox rabbis have to say about true Catholicism - whatever they say about Ratzinger or the Roman Protestants.
So, none of that. But I do think that Catholics can support the right of Israel to exist. I most certainly do. This doesn't mean that one supports modern, post-Temple Judaism or its exclusive claims going back to the 'chosen people' which were abrogated even before The Last Supper.
And again, this doesn't mean one supports that militant Judaism which roughs up Catholics or Orthodox in the old city just because some official or some troops had a bad day. And generally, the Israel military seems very composed, and professional, and prone only to receiving calumny from a leftist establishment, worldwide, that you might think would be almost unthinkingly in their corner. It doesn't even mean one supports their socialist ways, or various other things. It doesn't mean that one supports their destruction of holy places, should that be legitimately accused, any more than one would accuse the Palestinians prior to partition when the 'mandate' was in force.