Short answer from me would be... weight.
Long answer...I do not personally go out and search for
2S2P battery with ginormous amounts of capacity 'cuz of chargers not being able to see/read all four cells that make up a 2S2P pack. Also, I would rather have two 4000mAh packs than one 8000mAh pack. That's just me though.
I don't know what type of RCing you do, but I like to run buggies/STs/SCTs/RRs for about 10~20 minutes per run. I then grab another battery, another RC, and hit the track again. This way it gives me a chance at keeping temps down, and have a chance to do a quick once over for potential loose items and whatnots.