Uranus and Neptune are far - in orders of light-hours away. It took Voyager 2 nearly a decade to reach Uranus since its launch, costing almost $1B to build it in the first place.
It's not that we can't launch another mission, we're more than capable of doing it again, especially with the evolution of technology - but rather that it is ludicrously expensive and time-consuming, so NASA chose to allocate capital to other projects.
Even if we chose to launch another mission to the ice giants, expect up to a decade for development (depending on how fancy they make the probesprobe(s)), then another decade for arrival. Not very timely.