There are many reasons but quite often there is no one entity in control or solid, non-corrupt government to anchor to and build a relationship with without getting mired in an internal civil war between warlords. Also, the US generally operates in such interventions where we have direct interests such as Somali pirates impeding commerce at one point. Ukraine and Israel are happening in central zones of interest to the whole West. If we got involved heavily in each war it would be a worse mess. It's not that the left doesn't care, it's that we can't really help in a way we can afford and I don't see ANY country intervening so it's not as simple as left or right. Arms dealers are winning for now.