Because it's too late to start building nuclear plants. We need to be carbon neutral in twenty, at most thirty years, if we start a linear reduction to zero NOW. Building a nuclear plant takes at least a decade. When it's done we already used up all our remaining carbon budget.

And that's for the current speed at which nuclear plants are build. If we wanted to build an order of magnitude more, we'd first have to train nuclear engineers to staff them. And scale up production facilities for pressure vessels.