Activists say the reason for this is that they do not have the same juice within the Republican party as they do among Democrats, and have little ability to influence Trump’s policy approach. So they continue to focus on the party where their influence remains.
They weren’t “backing genocide”. They repeatedly called for a cease fire, hostage release, and two state solution. That’s 180° reversed from genocide.
Biden pretended to care whenever he was in front of a camera. Then he made sure Israel was protected and well supplied, and had all the support the US military could give them.