I suspect it has to do with a) the lifetime of the sensors required for this particular mission, and b) budget cutting. Long life spaceflight-rated components are extremely expensive. That's just a guess, though.
Its not the components. Radiation hardened electronic last for a long time. The limiting factor is probably fuel for the thrusters. Once you’ve run out of fuel, you can’t control where the thing is pointing. Hubble lasted so long because we refueled it every time NASA refurbished it in orbit.
It wouldn’t surprise me if there is a measurable torque from sunlight that will cause the telescope to slowly start spinning. The forces are tiny, but over a long time it can just mess up your day.