A. Because he's wanted for arrest, and you can't arrest someone over Google Hangout / Skype / etc.
I had thought that one possible outcome of the interview with him would be a decision to not arrest him. If that's the case it seems they'd want to get the interview out of the way by any means, and then if the need to arrest him stands, deal with that then.
B. Because the Ecuadorian government required guarantees the Swedish government cannot provide (because it'd be overriding the judiciary).
Interesting, I don't remember that bit, but that sounds likely. Oh well... what can ya do? shrug
The view of the Supreme Court of the United Kingdom was that the "interview" was, in practical terms, equivalent to arrest under English and Welsh law. From memory (though this was years ago!) a formal charge must be occur at the end of an interview. There's no distinction, in the view of the Supreme Court, between the interview and the arrest.
The argument that he's only wanted for interview was a large part of his defence prior to entering the embassy, and was entirely shot down by the courts.
B. Because the Ecuadorian government required guarantees the Swedish government cannot provide (because it'd be overriding the judiciary).