Because most western "leftists" are socdem coloured liberals.
They misunderstand or ignore key concepts like critical support, primary and secondary contradictions, materialism, and dialectics. Therefore it's no wonder that, even after a run of potentially good takes, they ultimately end up on the side of reaction.
For the actual MLs, some are outspoken but others are careful. I can only speak for myself in saying that I am genuinely concerned for my immigration status if I were to publicly speak out in favour of even the Palestinian cause in general, let alone Hamas and other decolonial forces.