[themal]

They probably turned it off for privacy. If you found an old Google profile URL, then you also knew the email address at Gmail, which would render Gmail useless for people who value their privacy.

Ideally, they need to offer the option to have a URL that is different from the Gmail address.

[corin_]

But in terms of code it's very easy to change the system so that it lets users pick their URL - the question is more whether Google will decide to do it, and the fact that they used vanity URLs in the old profiles suggests they may well.