I suspect the reason is because of the deep system integration of things like Safari, Mail, Contacts, and the like. Take Mail, for example, if you could set Sparrow as your default client on iOS you'd still have to configure Mail because any email sent from other apps uses the Mail API. Then rely on IMAP to sync those things up. Inelegant and un-Apple.
Having said that I can certainly appreciate why people might want to choose their default Browser/Mail client/Address Book/whatever. Hopefully Apple can sort that out, because I agree with John whole-heartedly on this:
The pressure for Apple to allow users to specify a third-party app as their default browser is going to increase significantly after this.