My understanding is they don't just want to question him, they're at the stage they want to charge him. You can't really do that over video conference.
EDIT: I didn't understand how Swedish law work. See AndrewDuck's reply. But still, they could interrogate him by videoconference (what Assange offered) or charge him in absence.
He hasn't been formally charged of any crime. If they want to prosecute him, they should charge him of something.
From Wikipedia:
"Assange has not yet been formally charged with any offence;[35] the prosecutor said that, in accordance with the Swedish legal system, formal charges will be laid only after extradition and a second round of questioning. The High Court found that the Swedish process has reached the stage of criminal proceedings, which would be equivalent to having been charged under English process.[36]"
There is nothing preventing a Swedish court from issuing charges in absentia, and there's nothing preventing them from carrying out questioning outside of Sweden. This is smoke and mirrors.
And it's something that has prompted a formal complaint against this prosecutor to the Swedish judicial ombudsman.
They can still interview him without laying any formal charges. As far as I know the interviews are generally held long before the formal charges are laid.
I'm extremely late to this thread and unsure where best to post this. That said, if I wanted to seriously question an individual, I would most want to control the environment. Would video conferencing allow that?
He hasn't been formally charged of any crime. If they want to prosecute him, they should charge him of something.