Think we were happy to keep both but both wanted to leave. Cech felt he could still play week in week out while Luiz, IIRC, was told his starting place wasn't guaranteed anymore and he decided to pack it in.
Mind you, when Luiz left last summer, he had 2 years left on his contract and was on bigger wages than Martinez is right now and yet, we sold him for only 8 million because he wanted to leave.