Was it not for real altruistic reasons?
Inflation is not good for anyone except holders of debt. In fact, inflation and deflation are both bad. Better to have a currency immune to all of those things. It also enhances its value as a safe haven.
Inflation is good for holders of debt, but it also tends to (in small amounts), help make sure that the economy is vibrant - since if the money you hold today will be worth less in time, there's an incentive to spend it. Deflation give the reverse incentive.
Now whether either of these situations is better is a different question - and my guess is that Satoshi feels that the problems with inflation are greater than those with deflation, and that's why it's that way. It's a philosophical decision - that's all.