Simple economics. It is cheaper and easier to build a slide with the dinette and/or couch than a slide with kitchen (sink, stove, fridge, water lines, drain lines, gas lines). Since most RVs have their main slide on the streetside (away from your campsite), then accordingly many (most?) will have the dinette/seating on that side with the more complex kitchen on the curb side.
My current A is a split; has half the kitchen (sink & stove, but not fridge) and the dinette in the streetside slide, with the fridge and a couch (opposite the dinette) on the fixed curbside.