Every country has “universal” jurisdiction in the sense that they can request the extradition of any foreign individual for any reason.
It’s then up to the rest of the world whether to grant that, or more specifically whatever country the individual happens to be in.
Extradition exists because otherwise crimes commited remotely across borders would be even more rampant than they already are, and it is in the interest of governments to allow other governments to prosecute individuals that commit particularly egregious crimes across a border, or escape across a border.
Whether Assange is one of those is debatable, but the US has a lot of weight to throw around and Wikileaks offended the government specifically. So here we are.