I think their goal was to make GitHub (and by extension git itself) more accessible to people who code, but are not familiar/comfortable enough with command line Unix or SSH to generate SSH keypairs. There are a lot of these people.

Although I agree that SSH keypairs feel more secure than a username/password, it would make setting up a new machine for development a lot easier than generating a new SSH keypair and adding it to my GitHub (or any other git server) account.