Some factors that cause normal wear and tear can be mapped out and predicted and are well established in the world of engineering. Things like thermal stresses, temperature cycling, battery charge cycles, etc.

One of the big challenges with space is that there is a bunch of stuff whizzing around that really can't be predicted for, and could cause major damage, or significant damage to localized areas. This would be things like space debris that's below the scale that nasa tracks, or that doesn't return radar signatures. Cosmic rays are like atom sized rail gun slugs that can cause serious damage to electrical systems and circuitry. For stuff like this, you have to work off what you know, and the outliers can be way off a normal distribution curve that you would expect for projected lifespan.