Assange could be questioned while he is abroad:

<snip>
In October and November Assange's lawyers offered a telephone or video-link interview (because telephone or video interviews with suspects abroad are lawful in Sweden and qualify for the purposes of a preliminary investigation), but the options were denied as Ny insisted that Assange be interviewed in person.
<snip>
http://www.thenewamerican.com/world-news/europe/item/11734-uk-supreme-court-denies-request-to-reconsider-assange-extradition-ruling

Why didn't they even take this preliminary step? That would seem to be the logical place to start.
They were hell bent on getting him on Swedish soil.