Supporters of boycotting Israel seldom focus on China, or Syria, or Zimbabwe, or other genuinely illegitimate regimes that violate human rights not in deviation from their own principles but systematically. This underscores their bad faith. Boycotters are not trying to send a specific message, such as "We object to your settlement policy in the West Bank" or "We think you need to be willing to give up more for peace." What they're saying instead is: "We consider your country so intrinsically reprehensible that we are gong to treat all of your citizens as pariahs." Instead of warning that Israel risks becoming an apartheid society if it fails to make peace, boycotters have concluded that Israel already is an irredeemable apartheid society. Like the older Arab economic boycott of Israel, which dates back to the 1940s, the cultural boycott is a weapon designed not to bring peace but to undermine the country.
Because Israel is a refuge for persecuted Jews, this kind of existential challenge is hard to disassociate from anti-Semitism--even if people like Meg Ryan and Elvis Costello intend nothing of the kind. It is for this reason that unlike in South Africa, where the internal opposition supported sanctions, none but the most extreme voices in Israel are likely to come around to the idea that their country deserves to be boycotted, divested from, or punished with sanctions. When people are trying to murder you because of your religion, it is difficult to credit the bona fides of those who merely want to shun you because of your nationality.