Because people have been conditioned over many decades to hate intelligent White people and side with dumb brown people.
The "Jews" in Israel - and America - are basically White Europeans pretending to be Jews even though they're genetically 85% European and only 15% Semitic.
Shalom.
Ashkenazi Jews (the Jews that evolved in Europe and intermarried with White Europeans) have the highest average IQ out of any ethnic group in the world.
A Zionist movement arose that advocated mass migrating back to their traditional homeland and creating a sovereign Jewish state centered around Jerusalem.
And that's exactly what they did in the early 20th century.
In 1948, the Jewish state of Israel was created with the blessing of the UN.
So basically, you have a bunch of White Europeans invading the Middle East, taking over land owned by savage muslime brown people, and then they did what White people always do when they colonize other countries: they promptly turned a 3rd world cesspool into an advanced civilization pretty much overnight.
No shit the anti-White, pro-brown Left is going to side with the a-rab muslimes.
It doesn't matter how barbaric the a-rab muslimes are and how civilized the Jewish Israeli's are.
In a conflict between White people and mongoloids, the Left automatically supports the mongoloids.
The Left also loathes successful people as evidenced by their hatred of the rich.
Don't forget that the Left are Marxists. And socialism revolves around demonizing and attacking the rich.
If you're rich, you didn't gain your wealth by simply being superior to other people. No, no. You did it by exploiting others, by cheating or you obtained your wealth by some other questionable means.
You see that with the Left all the time when they claim that the only reason White people are rich is because they enslaved brown people and stole their resources.
The White Jews are rich, smart and successful and the a-rab muslimes are poor, stupid and chronically fail at everything they do.
So again, by default the Left finds themselves naturally hating thriving Jewish Israeli's and sympathizing with the poor, poor "Palestinians".
Even though most Jewish Israeli's are themselves Leftists who created the kind of society most Western Leftists want to live in - complete with universal government run health care, abolition of the death penalty, gay rights, womens rights, a strong socialist "safety net" for the poor, etc - amd the a-rab muslimes violently oppose everything "progressives" claim to care about; it doesn't matter.
So strong is the Left's hatred for successful White people, it overrules any other mitigating factor in their worldview.