If the Swedish government legitimately wanted to question Assange about the risible "rape" charges (that they didn't even bother with while he was in Sweden), why didn't they give a guarantee not to extradite him to the US?
Because they want to extradite him and the "rape" case is an excuse.
Why not guarantee they won't extradite him to the US?
Well first of all, either an arrest warrant is valid or it is not. If valid, why do they need to bribe someone to obey the law? Sure, I sometimes give my kids snacks to bribe them into doing something they don't want to do, but I'm not the law.
Second, they cannot promise what their reaction would be. Suppose something impossibly-unlikely like the US has a video-tape of Assange killing someone in the US. The Swedes would then look pretty stupid at having given Assange a no-extradite promise. Now of course Assange isn't a murderer; the point is that the Swedes can't give blanket promises about extradition without actually seeing the facts of the extradition request first.
That's a nice abstract argument. The facts are otherwise.
If the US shows up with a video of Assange killing someone it will have been made on a Hollywood renderfarm.
We know that because we know that the US is bent on hunting Assange down for political reasons and have set up a secret Grand Jury for this purpose.
When the law says one thing and justice says another, the customary thing to do is to push the law towards justice, not the victim of injustice towards the law.
> "If the US shows up with a video of Assange killing someone it will have been made on a Hollywood renderfarm."
I remember when that idea was just the plot of a mediocre Michael Crighton book. That it doesn't strike me as absurd anymore is either amazing or frightening. How would you even begin to fight that kind of evidence in front of a jury?
Because they want to extradite him and the "rape" case is an excuse.