The problem with these long term journeys is that it’s entirely possible that the probe could get half way there by time we develop the technology to make another probe that’s twice as fast and cheaper. Or maybe we make other discoveries and find that we don’t need the data the probe is equipped to gather. We’re not really near the limits of propulsion and space engineering yet so it doesn’t make a ton of sense to invest in something with such a distant payoff when it’s somewhat likely to be outdone before then.