Because the UK was reluctant to give extradition based on the conditions offered by the US. Part of the reason the US is offering a plea deal is that it bypasses the need for extradition. Australia also asked the US to drop the case.

So offering a deal only to have the UK agree to release Assange and lure him to US territory would definitely be a diplomatic issue, possibly jeopardizing future extraditions from the UK, for instance.