Weeeell, maybe because Sweden is a democracy and has no "authorities" that can guarantee the outcome of hypothetical future legal proceedings.
Also, note that Assange was obviously in Sweden when the rape allegedly happened, and only became conveniently afraid of extraditiom to US after those allegations surfaced.
An extradition is not a legal proceeding, it's a diplomatic one and being a democracy they can very well agree not to engage in a diplomatic exchange beforehand.
Well, I don't know what treaties are to the United Kingdom and Sweden, so I can't speak to the relevant matter, but there is a general sense (insofar as "international law" can be thought of as an actual concrete thing) that treaties are laws. To the United States, for example, treaties are explicitly treated as laws (see http://law.justia.com/constitution/us/article-2/18-treaties-...). Most other countries take their treaty obligations very seriously, although the actual mechanics depend on each country's legal/political system.
Source: http://sverigesradio.se/sida/artikel.aspx?programid=2054&art...
The allegations surfaced in August 2010, four months after Collateral Murder and one month after the Afghan War Diary was released. Also earlier that year the diplomatic cable releases began, and while most hadn't been released yet by August, some had, and they were a recently-known quantity by then.
The timing of the allegations was awfully convenient. Far more "convenient" than Assange's paranoia.
[1] http://www.bailii.org/ew/cases/EWHC/Admin/2011/2849.html
Onward extradition would have been far more complicated via Sweden as the UK would have had to approve it as well. Why not just have the US extradite Assange from the UK to the USA directly? If the CIA was going to concoct a rape claim why not make it a little more than "she was asleep"? If the USA is ruthless in the pursuit of its enemies (except of course Snowdon is a US citizen who, rightly or wrongly, very much did break US law) then why has it been so decidedly non ruthless in the pursuit of Assange?
Enemy of my enemy, and all that.
The "Chinese" certainly don't have too hard of time getting good info (OPM hack, etc), at least going by the way you hear the cries of fire in the crowded theatre of what passes for journalism and news.
If you think of Sweden as ABBA, think again:
"In December 2010, The Independent revealed that the two governments had discussed his onward extradition to the US."
"The rendition and subsequent torture of two Egyptian political refugees in 2001 was condemned by the UN Committee against Torture, Amnesty International and Human Rights Watch; the complicity and duplicity of the Swedish state are documented in successful civil litigation"
http://johnpilger.com/articles/assange-the-untold-story-of-a...