Mamdani won by focusing on affordability. But crucially, he also made a very clear and concise plan for HOW he was going to accomplish the things he wanted to. He had a whole series of bills he could cite that were currently being held up by the existing mayor, and he had a clear path to accomplishing what he promises. Democrats promises are mostly just vague platitudes. They often would require impossibly large margins in the Senate. They make no plans for bypassing the filibuster or hostile rulings by a corrupt Supreme Court. And they have no clear plan for how they'll avoid the "rotating villain" problem of their always being a Manchin or Lieberman there to sink positive legislation.
They need to focus on affordability AND make a clear and convincing case for how precisely they can achieve those aims. The reason no one believes them is because they never have any real plan on how to carry through their impressive-sounding agendas.