I'm no expert in Thai language but it's my understanding that คือ and เป็น have the same meaning in this context, as far as I'm aware. So why not use them interchangeably?
Trying to think of an English equivalent example from the top of my head (and I'm sure there are better examples) I could say something like "My girlfriend went for a walk into town, while I wandered down to the beach". Same meaning; it just makes the sentence a little less robotic sounding.
As I say, I don't consider myself any kind of Thai language specialist (for sure!), so I'm happy to be corrected.