@blueshoes
The Times of Israel provided various reasons in the articles:
The one given by Netanyahu was:
“time and again…weapons that we supplied in one battlefield end up in Iranian hands used against us.”
However the Times of Israel also said:
"Earlier that day, The New York Times reported that Russia has drawn down forces in Syria and removed the sophisticated S-300 anti-aircraft system that has been a major threat to Israeli Air Force operations in the country.
The development could open the door for Jerusalem to upgrade its level of support for Kyiv, as Russia’s presence in Syria has been a major consideration for Israel’s position on the matter.
A senior Israeli defense official and two senior Western diplomats told The Times that the redeployment would decrease Russian leverage on Israel and that this may lead Jerusalem to reconsider its support for Ukraine.
The Israeli position is believed to be based on its desire to maintain freedom of operations in Syria as well as to avoid causing problems for Russia’s large Jewish community. Israel is also concerned that as Russia pulls out its military presence in Syria, Iran will be able to increase its influence.
Moscow has already moved to curb Jewish institutions in the country: Over the summer, Russia’s Justice Ministry filed a petition to liquidate the Russian offices of the Jewish Agency for Israel — a semi-governmental organization that encourages and facilitates Jewish immigration — in what analysts view as a reaction to Israel’s criticism of the invasion.
...
On Monday, Dmitri Medvedev, the deputy head of the Russian Security Council, warned Israel in a social media post that any “reckless” supply of military equipment to Ukraine “will destroy all interstate relations between our countries.”
Israel has in the past provided humanitarian aid to Ukraine, including operating a field hospital for several weeks in the early days of the conflict, and also protective military equipment such as helmets and flak jackets. More recently, however, it has also provided the Ukrainians with intelligence about Iranian drones, according to Ukrainian and Israeli officials, who say that Israel has also offered to examine the remains of drones that crashed."
Suggesting that the real reasons are: a) to protect their forces from Russian and Iranian attack on their border, but, perhaps more fundamentally that b) that the Jews in Russia are effectively being used as hostages. While the US may have the odd person (eg basketballer) in Russia being held hostage, there aren't that many. Estimates are that there are at least 150,000 Jews in Russia. In Russia, Jews tend to be the classic scapegoats when things aren't going well - and certain members of the current Russian authority have made it very clear that they are linking the treatment of Jews in Russia to Israeli behaviour. Neither the US nor Ukraine are in any position to help if the Russian authorities turn on the Jews.