The sexual assault charges were dropped a few years ago.Two different things.
Assange should face the sexual assault charges for sure but the USA want him because Wikileaks exposed a load of crap being done by their military and covered up i.e whistleblowing.
If the USA can grab anyone across the globe for such whistleblowing it'll be a blow for investigative reporting.
The US charges are not for whistleblowing, they are alleging that he encouraged / assisted Chelsea Manning to release state secrets (which if true would be a criminal offence, whether ethically correct or not). The UK is seeking assurances that he would not face the death penalty as it does not extradite under those circumstances.
The (potentially very real) concern is that he will be extradited on these charges which do not carry a risk of the death penalty and then suddenly face more, heavier charges upon arrival in the US.
Its a bit far fetched but then so is a man being evicted from diplomatic immunity and having his citizenship revoked for not cleaning up after his cat.
I can't say I necessarily like Assange and personally think he probably is a bit of a tosser, but this does have the hallmarks of international pressure from the US on the Ecuadorian government.