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.
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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]
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Also, it would have cost vastly more to put full high speed lines required in most of the way to London Waterloo.
From: http://en.wikipedia.org/wiki/High_Speed_1
" 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] "