Das war tatsächlich laut "offizieller" Stellungnahme von Ray Dillinger der Gedankengang:
I remember this discussion, actually.
Finney, Satoshi, and I discussed how divisible a Bitcoin ought to be. Satoshi had already more or less decided on a 50-coin per block payout with halving every so often to add up to a 21M coin supply. Finney made the point that people should never need any currency division smaller than a US penny, and then somebody (I forget who) consulted some oracle somewhere like maybe Wikipedia and figured out what the entire world's M1 money supply at that time was.
We debated for a while about which measure of money Bitcoin most closely approximated; but M2, M3, and so on are all for debt-based currencies, so I agreed with Finney that M1 was probably the best measure.
21Million, times 10^8 subdivisions, meant that even if the whole word's money supply were replaced by the 21 million bitcoins the smallest unit (we weren't calling them Satoshis yet) would still be worth a bit less than a penny, so no matter what happened -- even if the entire economy of planet earth were measured in Bitcoin -- it would never inconvenience people by being too large a unit for convenience.
Ich finde diese Erklärung viel plausibler:
Satoshi cared that people using Javascript (?) or other languages which encode all numbers as 'double' (64-bit floats) would not have to jump through hoops to avoid stupid accounting mistakes.
Your 64-bit float has 52 bits of mantissa, so, in order to avoid rounding errors ever going the wrong way, the number of units involved in bitcoin-related math must never be more than 251, which is 2251799813685248 units. 21 million coins times 108 divisions (Satoshis as they are now called) is 2100000000000000 units - comfortably just below the limit allowing "simple" accounting implementations in such languages to be accurate.
And as Hal Finney pointed out in 2008, even if the entire M1 money supply of the world as of that time were expressed in Bitcoins, the smallest division would still be worth less than 1 USAmerican penny, so there is no need for more than that many units.
Also es hatte einen mathematischen Hintergrund. Als ob Satoshi nicht wusste, dass sich die Geldmenge ständig verändert (mehr wird).