Besides anything else, part of the reason is that many (probably most?) Republicans don't in fact actually support Israel in the first place. What they actually support is messianic right wing Christian Zionism (with emphasis on "Christian").
They support Israel because they believe it is necessary as a prophesized step in order for the world to end and be destroyed (and in particular Israel being destroyed with the Battle of Armageddon taking place in Israel).
https://www.washingtonpost.com/news/politics/wp/2018/05/14/half-of-evangelicals-support-israel-because-they-believe-it-is-important-for-fulfilling-end-times-prophecy/https://www.thedailybeast.com/inside-israels-unholy-alliance-with-right-wing-evangelicalsetc etc etc
It shouldn't be too hard to understand why many Jews and many Israelis might be somewhat uncomfortable with that sort of "support."
Of course, that is not really true of all Republicans, just the ones that genuinely believe in end times evangelical Christianity. So, for example, Trump likely doesn't believe in this because he doesn't have any particular religious beliefs or moral code, except insofar as "what is good for me personally is good" is a religious belief system or moral code. He just supports Israel in this way because it is convenient politically as a way to lock down support from members of the Republican base who do believe all this right wing religious lunacy.
