Because, as I'm sure you're aware, Mr. Assange is not on British soil.
He is on British soil. Britain, like most (all?) countries in the world, doesn't consider embassy buildings to be the soil of the embassy's sending country.
For countries that have their embassy in the UK, them asserting that the embassy is their own soil doesn't have any effect. Sweden still needs to deport Assange from the UK as that is where he is as far as the UK is concerned. The Swedes could start extradition proceedings with Ecuador if they wanted but the UK would arrest him when he left the embassy and the extradition from Equador would have no effect. The UK doesn't need to extradite him from Equador either. They just need Equador to give him up to UK authorities.
The slightly more interesting case might be a UK embassy on the soil of a country who does say that embassies are the soil of the sending country. Because there are probably a few cases where it could matter - e.g. accident insurance that applies only in the UK - i.e. not for travel. If you fell and broke your leg in a UK embassy somewhere else in the world could you claim? The UK courts would probably say no even if the foreign country said "actually that building is in the UK".