Because the Ecuadorian President decided to offer him sanctuary just to yank the US's chain. They couldn't care less about him. I see the situation developing in one three ways:
(1) Assange gives himself up to the British authorities who will ship him to Sweden
(2) Britain gets tired of the abuse of diplomatic privileges and tells the Ecuadorians to give him up or GTFO. At this point they could probably give Assange a diplomatic passport and take him back to Ecuador without him being arrested.
(3) The Ecuadorians tire of their embassy guest and give him notice to find a new home. He could try his luck with Russia or, if he has any sense, go to Sweden.
Of course Britain could just forcibly extract him any time they wanted. The SAS has a team permanently based in London and would enjoy a nice day out. However I'm sure this option has been categorically ruled out due to all the diplomatic fallout that would ensue.