Ok, let's get this down straight.
The Guardian, 2017-04-21: The US attorney general has explicity said that getting Assange is a priority.
Sydney Morning Herald, 2012-09-27: The US military has declared Assange and Wikileaks an "enemy of state".
Independent, 2010-12-17: After 7 months of isolation detention, Manning is offered a plea bargain if (s)he, erm, voluntarily names Assange as fellow conspirator.
Wikipedia: The USA has since 2003 a completely one-sided extradition treaty with the UK which allows UK citizens to be extradited without contestable evidence, but not US citizens in return.
Wikipedia: Gary McKinnon was a Scottish hacker which hacked himself into US systems. After the 2003 treaty McKinnon was told that seven charges are held against him, each worth 8-10 years prison, but he might get only 3-4 years at all if he, erm, voluntarily travels to the US. No guarantee was given. Given that the government very nearly extradited one of their own citizens, Assange fears are fully justified.
The Guardian, 2018-02-11: Sweden does not want to press charges anymore since 2013, but the Crown Protection Service tried to sway them. In an answer that refers to the suggestion that the case could be closed as early as August 2012, the CPS lawyer is quoted as: “Don’t you dare get cold feet!!!”.
The Guardian, 2013-07-03: A hidden microphone was found 2013 in the embassy where Assange resides.
The Telegraph, 2013-05-21: Officers of the GCHQ itself said in internal e-mails that the charges against Assange reek like a fit-up.
The Guardian, 2017-11-10: The CPS admitted having destroyed evidence, notably emails in 2014!
The Guardian, 2017-05-19: The Swedish prosecutors no longer want extradition as of 2017. The international warrant of arrest has been revoked. There are no rape charges anymore.
The Guardian, 2018-02-13: The whole affair is now that they charge Assange with skipping bail for an extradition request that does not exist anymore.
For further information I refer to the excellent answer by James Wood who presented the ruling.
There are still many people who are claiming that the UK only follows the law, so I have researched if there is something, you know, unusual about the behavior concerning Assange. You know, something which indicates that Assange is not treated like a common criminal.
The Guardian, 2012-08-15: The UK threatened to enter the Ecuadorian embassy by force. AFAIK no country has ever dared to speak out such a threat before, not even the USA/USSR during the Cold War.
Prize question: What have Aung Sang Suu Kyi (Burma), Anwar Ibrahim (Malaysia) and Jason Rezaian, Washington Post journalist (Iran) in common? They are all people which have been detained and where the Office of the United Nations High Commissioner for Human Rights pressed for their release. In all these cases both Sweden and the UK supported the decisions. Now the very same Working Group for Arbitrary Detention came to the conclusion that Assange is detained unlawfully. The UK ignored the decision.
The maximum sentence for skipping bail in the UK before a Magistrate Court is three months, the alleged sexual assault is insofar irrelevant for the UK as it is Sweden's responsibility. As the London Metropolitan Police has clearly stated since a while, they have problems with rising criminality and budget cuts. While the article is from 2017, an overview of other earlier articles indicate that Londoners know that shoplifting, theft, burglary and assault is less and less prosecuted or investigated. But Assange's alleged sexual assault and bail skipping must be very important because the Met surrounded the embassy with up to 50 (!) policemen 24 hours a day, 7 days a week which until August 12th, 2015 summed up to a notable 12.6 million pound bill. One commenter pointed out an article in which the Met claimed the costs were covered by the budget for diplomatic protection. The little problem with this answer is that there is a responsible unit, the Diplomatic Protection Group (Special Operations 6), now the PaDP, which is recognizable by driving red cars and being heavily armed. But the policemen guarding the embassy are not part of the group because they are missing the heavy armament necessary for the service. And against whom should they "protect" the Ecuadorian/Colombian embassy exactly?
So the claim that the UK is "just following the law" is baseless. The claim that it is about sexual assault is baseless.
And now someone can tell me why it is not unusual that a government pays millions of pounds to track a fugitive (give a counterexample, please), that evidence is destroyed on purpose and that after the original country has dropped the arrest warrant, the UK judge admits that the British arrest warrant is now solely for skipping bail. They explicitly say that seven years in an embassy is an insufficient "punishment" for skipping bail.
Based on the former conclusions the reason is obvious for me: The US government has either strongarmed the UK that they want Assange and threatened repercussions or convinced the UK that sooner or later Wikileaks will publish dirty laundry about UK politicians so they should persecute him in their own interest. Possibly both.
I adjusted this addition now because there have been now clarifications posted on other answers. Please see the edit review to see the old version.
Some claimed that the charges are not dropped, this has been refuted now. I also reviewed the headlines and the text again.
At some of the more hostile commenters: Knowing that it is a hot topic, I deliberately sourced every statement on which my conclusions are based.
In your accusations, claims and scorchers you did not come up with a single sourced statement of your own. No word about misbehavior on the government side (destroying evidence, remember?). It would have been given a much better impression if you were providing facts and evidence to prove that I am wrong, so I believe your actual behavior has bolstered my case. Thank you.
Have now reworked the answer again and put some other info together.
A commenter had remarked that the selection of my sources is rather limited. I absolutely invite everyone to make their own research and look out for other sources to get a more rounded view. Note what they claim, what sources are presented and what conclusions are made. Look out for things who may be misrepresented here or contradict your own sources.