An issue is that the track layout is such that it would only have been useful for the abandoned "North of London" services- or HS2 through trains, before that link got cancelled. It is of very little use as a "budget" type terminus (think SNCF TGV Ouigo) as to reverse requires either multiple reversing moves to shunt across, using the Temple mills depot link part way through, or to run all the way to St Pancras and back. If it had been built with a reversing siding accessible from both international platforms to the west of the station it would potentially have been useful.
But ultimately, it is this country's insistence on secured borders that made the idea of services beyond London unworkably expensive and thus killed the utility of Stratford as an international station.