Perhaps if I’m ever granted the opportunity to present my case before NASA and Congress, I might offer a more well-thought-out explanation. But right now, “because it would be awesome” should suffice.
Orbiters for one or both of our ice giants ought to be much higher on our space exploration priority lists than they seem to be.
And that's assuming that 5W suffices: "New Horizons" has 200W and a 2 metre diameter radio dish, and still took 15 months to download the data from its brief Pluto encounter (although at a slightly greater distance than Neptune's orbit)
If you want it to be an orbiter rather than a brief flyby then it needs some way to shed a lot of velocity: either a lengthy rocket burn and/or aerobraking by grazing Neptune's atmosphere.
So unfortunately it seems well beyond cubesat terrain: even a "cheap" probe to the outer planets is a large and complex machine.
AFAIK solar sails aren't able to slow down, so you'd need another form of propulsion to establish orbit.
Sometime around then, we also got a computer modem, and one of the first things we did with it was find some NASA BBS where you could download images from Voyager 2. At a blazing 300bps. The long-distance charges were probably more than what I pay for a week of gigabit internet service at home now.