Most materials contract when they cool down. For example, differential contraction between metal contacts and semiconductor materials can cause them to detach or break, disrupting electrical connections. Also extremely low temperatures can lead to changes in semiconductor properties such as carrier mobility, which affects how efficiently electronic signals are processed.
Couldn't all connectors be made with some kind of expansion joint, like how infrastructure (bridges etc) are made? Given how they use older nodes, this may perhaps even be possible for transistors (Assuming they also have such expansions)?
Normally materials in chips are selected so that they have similar thermal expansion coefficients - otherwise they would fall apart just from reaching normal work temperatures.
There's another problem here: below a certain temperature semiconductors become insulators. You're running the risk of your chip shutting down in a disorderly manner.
Of course you can do all sorts of things to make the electronics able to survive, but ultimately that supposedly just wasn't part of the initial design requirements they settled on.
It's good to remember that the total lifetime budget of this lander is only $121.5 million, and that includes all the staff still receiving data. It was always intended to be a technology demonstrator, not an ongoing science lab.
