Just because it's the right thing to do doesn't mean it wins elections, and anything to do with cars is almost impossible to address rationally. Starmer, who is desperate not to mess up before the general election, so he can get into power and change things for the better, is butting up against Sadiq Khan, who is safely in power, and already has the popularity to make these kind of tough decisions.
This isn't the same as not interrupting - The Tories wanted this byelection to be a proxy referendum on ULEZ, and would have loved for Labour to engage more on this issue. They were quite happy to draw as much attention as they could away from anything positive Labour had to offer, and instead focus on an issue where they know they can "win" the argument, no matter the facts.