I've had both of the JRXS sedans, and the battery is very easy to get into and out of the car. Two screws and the battery litterally falls out the bottom. The Type-R is the only touring car (maybe the Magic, too) that doesn't need to be rebalanced when swapping between different weight batteries, as the pack is always centered between the front and rear axles, as well as being centered across the width of the car. Even the F/R bias of the original mid-motor JRXS is affected by running batteries of varying weight. As for why that car didn't take off, it was partially due to the short arms which narrowed the setup window beyond what most racers could manage. I hacked my way into the B-main at Snowbirds with one, so with a better driver it was definitely worthy of an A-main appearance. However, just as soon as the car was released, the factory team began running protoype rear motor cars, the aforementioned Type-R. That complete lack of confindence from the team drivers (mainly in mod class) was surely a big reason many club racers didn't bother buying the mid-motor car.
As for why most touring cars evolved into having the batteries on the right side.... going back to the dawn of competitive touring car racing, the most successful cars had saddle packs. The packs were a pain to load into discharge trays and newer chassis designs like the Losi Street Weapon and AE TC3 proved that running the batteries all on one side or the other could be properly balanced, left to right. Once shaft drive cars lost favor, the now obligatory dual belt design was continually "optimized" (get used to hearing that word
, I probably owe Xray $6.99 for typing it) with an ever lower center of gravity. The top decks were constantly being lowered right up to the point in history were Lipo batteries gained favor. Since Lipo dimensions were essentially fixed and were too tall to fit under the top decks of even recent designs (-2008), most manufacturers began seperating the top deck from the other myriad bulkheads in an effort to fit Lipo packs and begin allowing racers the option of adjusting the flex characteristics of the chassis itself.
Here's a better question which I don't have even a foggy clue of an answer; why are most if not all C-hubs and spindles geometric copies of one another?
