Why are NS buying single decks for new trains these days?
Accessibility and (with the sprinter trains) dwell times.
Regarding the ICNG: the requirements were: an EMU suitable for the High Speed Line, with accessible access, and proven technology. When the tendering process began, there was no solution for same level boarding with the Dutch platform height of 76 cm with a double deck train, which, as a result, meant that the train would be single deck.
As they replaced single deck trains and offered a higher capacity than the trains they were replacing, this wasn't seen as a problem at that time (and bear in mind, that at the time when the ITT was written, the government still had the plans to abolish the free travel card for students, which would cause passenger numbers to decline).
Nowadays, there are solutions in place for multiple units with both single and double decker coaches (Bombardier Omneo, Siemens Desiro HC) or even on fully double decker trains (Stadler KISS), so when the tendering process for the Double Decker Next Generation begins (those are expected to be in service in 2027, and will replace the DDZ trains), there will be solutions available for the accessibility problem.
Regarding sprinter trains (e.g. Sprinter Next Generation), the dwell times are too important. Sprinter trains should be able to have a maximum dwell time of 42 seconds, including doors opening and closing, and the accessibility requirements are there as well. Double decker trains are usually not be able to match those dwell times.
I personally don't expect the next generation Sprinter trains (Sprinter 5th Generation, expected to be in service in 2025), to be (fully) double deck due to those requirements, but it would be a nice surprise and a welcome boost in capacity.