The curious thing is that a new line was built from Schaerbeek (North Brussels) to Mechelen, but it is scarcely faster than the old main line. A missed opportunity to build a high-speed line to Antwerp via Brussels Airport I think - one wonders if this would have been better value than the lightly used high-speed line to Aachen.
1) there is no high speed line between Brussels and Antwerp because the land in this part of the country is either almost totally built-up or used by high value businesses, so appropriating all the needed land would be hideously expensive. It would also be extremely unpopular, so no flemish politician would ever take the risk. So you are left with the existing lines only.
2) there is a new line between Schaerbeek and Mechelen because it was possible to build it on the central reservation of the Brussels Antwerp motorway. This central reservation was initially foreseen for a superfast motorway that has never been built. However, it doesn't go all the way to Antwerp and ends in the middle of nowhere, with no easy connection with other railway lines. So this new line will never go further north than Mechelen.
3) the situation and configuration of the station of Mechelen imposes a maximum speed of 120 km/h to traverse the station. At the exception of the HSTs, all trains stop there. A top speed of more than 160 km/h on the rail lines north and south of Mechelen will only save a few minutes in the total journey.
4) Belgian population has clearly expressed their desire to see the public money invested in railways used for everyday's life, not for the advantage of international travellers. So there won't be any further developments on the belgian high speed network. All available funds are now directed to renovation of stations, increase of capacity, and increase of speed only when it benefits to domestic travel.