"Given that calculators were probably most commonly used in finance initially, I would guess that the most common number used (possibly even now) would be 0. Placing that most common number at the thumb position has clear utility, similar to that of the spacebar."
To expound on this a bit, the design is not arbitrary. In finance digits don't actually occur with equal frequency. 1s are far more common than 9s for example. See Benford's Law for more info. Its used in forensic accounting to help detect cooked books.
I use a '10 key' for a couple hours doing books most workdays and it would definitely be less convient if the numbers were flipped. Having said that I don't have a problem with phone keypads. I think this might be because phone dialing is almost exclusively done with thumbs on modern phones so you don't use the same muscle memory you do when using a '10 key.'