As Expatriate wrote below (29.11.20), this is one of the reasons: "The CX has more brightness than the BX. If you have a bright room, the CX would be better. If you have a dark room or watch mostly in the evening, the BX will do."
Another reason is the better processor: the CX has an Alpha 9 Gen 3 AI image processor, the BX Alpha 7 Gen 3 AI image processor. Also, the CX's stand is wider, so it can "spread" the sound into the room better. I think the BX is more for hanging on the wall. That's why I bought the CX and I'm very happy with it.
Here is a detailed comparison in German:
https://www.zambullo.de/fernseher...