Well, more state in code usually makes it more difficult to do things like run the code concurrently. You have to worry about managing data races when there is a lot of shared state, whereas in stateless code no complex managing is needed
Although this is true of stateful code, I think an even more fundamental, but related, reason to reduce state is this: code that is stateless always behaves the same way so it can be characterized and reused more easily than code that changes behavior depending on the state. This is the reason it is good for concurrent programming, but it also means it has a more concrete/consistent nature.
