In addition to what others have mentioned, there's also a problem of communication. Inverse square law is a bitch. It was actually assumed at the start that the limit of the Voyager missions would be communicating with the probes, but improvements in radio technology have kept it going longer.
Information on the heliopause is about the only useful thing we can get from something out that far. It turns out to be a lot more complex than we thought. After that, there's nothing interesting until you can get to the next star, and our radio technology isn't up for that.