The target markets are very different. Ashford really just appeals to those from East Kent; it did very little inwards traffic despite all the French tourists one encounters in the area, particularly Canterbury. Ebbsfleet even more so, its users almost wholly arriving by car, from surprisingly far away (I encountered Cambridgeshire residents there), and also from a lot of NE/SE/outer London. Major Eurostar flows of those coming from the Continent, and a notable quantity of touring Americans, head wholly to St Pancras, along with all from provincial Britain who come into London by train, and these groups may well be a majority of those on Eurostar. They are actually quite different markets.
The directional flow issue is similar to that experienced by international airlines. From the UK to the USA there is a reasonable demand from provincial areas, but a plane takes everyone, and those Americans travelling in the opposite direction are almost entirely travelling to London - Edinburgh is apparently the single UK outlier in this.