I haven't seen any plausible reason why the prosecutor couldn't question him in London, especially given the exceptional circumstances of the case.
edit: Yes, I have seen the reasons put forward by the SPA mentioned in the article, but these complications were easily foreseeable and it is clear the Swedish prosecutor did not allow enough time to resolve them. Indeed the prosecutor claimed until this march (at which point she changed her mind) that it would be completely impossible to interview Assange at the embassy.