Because it isn't foggy anymore.
'Londinium' is nowadays called London.
It is called its name.
London Fog is so thick because of pollution. The fog, when mixed with the foul pollution of the area, becomes so thick it has another name (pea soup fog). This is why London Fog is so thick. London hasn't had this type of fog (smog) for around 50 years
London is the capital city of Great Britain (better known nowadays as the United Kingdom).
One only and that is called london City
London
London is the capital city of England. There is also a city called London in Ontario, Canada.
London, England is the capital city of the United Kingdom but there are other cities called London in other parts of the world.
The City of London and the city called London are two distinct places. The head of the City of London is the 'Lord Mayor of the City of London'. This should not be confused with the 'Mayor of London', who is in charge of the whole of Greater London.
There is no city or town called 'London", but there are a lot of "London streets".
It is the 'City of London', usually just called 'The City' or 'The Square Mile'.
Londinium is important today because it is the capital of Great Britain. It is the present day city of London.