Satoshi discussed this here:
Satoshi, I figured it will take my modern core 2 duo about 20 hours of nonstop work to create ฿50.00! With older PCs it will take forever. People like to feel that they "own" something as soon as possible, is there a way to make the generation more divisible? So say, instead of making ฿50 every 20 hours, make ฿5 every 2 hours?
I thought about that but there wasn't a practical way to do smaller increments. The frequency of block generation is balanced between confirming transactions as fast as possible and the latency of the network.
The algorithm aims for an average of 6 blocks per hour. If it was 5 bc and 60 per hour, there would be 10 times as many blocks and the initial block download would take 10 times as long.
It wouldn't work anyway because that would be only 1 minute average between blocks, too close to the broadcast latency when the network gets larger.He highlights latency of one minute being a main issue with a large network size. He also highlights being concerned about the total block size.
Bitcoin has a longer latency than possible because of how nodes request information, as he discussed in the mailing list on 17-11-2008,
The inventory-request-data scheme introduces a little latency, but it ultimately helps speed more by keeping extra data blocks off the transmit queues and conserving bandwidth.
Also related, in a letter to Mike Hearn, 12 Apr 2009, Satoshi wrote while discussing fast micropayments:
My choice for the number of coins and distribution schedule was an educated guess. It was a difficult choice, because once the network is going it's locked in and we're stuck with it.