Besides consumables like fuel, mentioned nearby, there are also mechanical parts like gyroscopes or reaction wheels (https://en.m.wikipedia.org/wiki/Reaction_wheel) which maintain pointing. If the s/c contains an imager, there will probably be a mechanical shutter, protective cover, and/or filter wheel.

There can also be higher power electronics, like signal amplifiers or radios, which tend to fail more often than computer electronics. Finally, there is cumulative radiation damage, and the possibility that a combination of single event upsets can get the s/c into an unrecoverable state.

From time to time, operational changes can force the s/c into new operating modes ("we need to flip the camera to take images to fit a new point spread function"). These new operating modes can cause unforeseen consequences ("when flipped, the antenna has to be pointed differently to target the ground station") that ripple through the system. As the mission wears on, the chance of a new operating condition tickling a latent problem increases, because there are a lot more latent problems.

Another aspect is that many spacecraft use thruster fuel over time, both for pointing and for station-keeping. Once you run out of fuel, the spacecraft may lose the ability to keep its high-gain antenna pointed at Earth, so you may lose touch with it completely, especially for deep space missions; but more seriously, it may also stop being able to keep its solar panels pointed towards the sun.

(Some spacecraft don't use thruster fuel for pointing, so they don't suffer from this so badly.)

In addition, if your vehicle is in Earth orbit, it may be assigned a position. For example, geostationary satellites must keep within their assigned slot or else you'll be very unpopular. (See, for example, the Galaxy 15 zombiesat, which went nuts and started drifting into other satellite's slots, while broadcasting hash.)

So if you run out of fuel, you won't be able to do this any more, and as a result some satellites are required to keep a fuel reserve so that they can be sent into a safe graveyard orbit at end-of-life.