There is certainly a political element, when they tried doing cheaper missions they had two failures in a row which was really embarassing, even though probably

There is certainly a political element, when they tried doing cheaper missions they had two failures in a row which was really embarassing, even though probably if they had stuck with it it would have still worked out cheaper than using the low risk approach. Mainly though if you design a spacecraft to have a 99% chance of lasting five years it ends up with a pretty high chance of lasting 30 years.