The Erostar terminates in North London so that the vast majority of its passengers who are from north of London don't have to get across the city. Despite heading in the wrong direction its quicker than the underground.
You have ebbsfleet and Ashford for passengers from south of the city.
Eurostar terminating in North London is because they wanted the high speed line to run to the East of London so it would be easier to connect it to the North of the UK for further possible direct links from Manchester/Liverpool/etc.
Also, it would have cost vastly more to put full high speed lines required in most of the way to London Waterloo.
"
The next plan for the Channel Tunnel Rail Link involved a tunnel reaching London from the south-east, and an underground terminus in the vicinity of Kings Cross station. However a late change in the plans, principally driven by the then Deputy Prime Minister Michael Heseltine's desire for urban regeneration in east London, led to a change of route, with the new line approaching London from the east. This opened the possibility of reusing the underused St Pancras station as the terminus, with access via the North London Line that crosses the throat of the station.[26]
"
I'm having a hard time understanding your justification. Ebbsfleet is 25 miles ESE of London (Ashford 50+ miles), with no local (TfL) connections. I'm also curious to know the basis for your claim that "the vast majority" of Eurostar customers are from north of London. I hear lots of people live in Surrey, not to mention London proper.
My point is that London is the south east, by bringing the erostar round to north London it is easer for the whole country to use. There is probably an argument that they should have a station in South London but the Erostar is there to cater to the whole country, not just london.
You have ebbsfleet and Ashford for passengers from south of the city.