I agree with Yatsey. Also, the capacity to increase the number of available bitcoins would cause inflation, which is something Bitcoin is trying to avoid.

edit: To add some weight to Yatsey's opinion, there was 1.23 Trillion in USD circulating globally as of Jan 15th. That's 1,230,000,000,000 dollars. Considering cents, you have a possible 123,000,000,000,000 units of USD in circulation. Compare that to 21,000,000,000,000 uBTC. If we ever get to the point where BTC is accepted as much or more than USD, a single BTC, or even uBTC, will be worth so so much.