Nuclear fission reactors have a lot of moving parts, and creating something that lasts 50 years without one of those moving parts failing, unmanned, and subject to the harshness of space, isn't trivial.

Nuclear batteries are extremely reliable though, as they are fully solid state. A Pu-238 battery can emit useful amounts of power for upwards of a century, and there is virtually nothing to break.

The Russians have a few of those: https://en.wikipedia.org/wiki/BES-5