In not sure you appreciate what it takes to keep a VAR system in working order (and in compliance with the technical specifications).
Grounds with VAR systems that are going to be used during the season have to go through fairly thorough, time-consuming and relatively costly checking and calibration procedures before the beginning of the season.
Then they have to be re-checked (albeit in a less in-depth procedure) before each game. If faults or mis-calibration are found, those issues have to be rectified.
All the equipment, manpower, time and logistics needed for the pre-season and pre-game checks are allocated and budgeted for the teams in the Premier League only.
The resources needed to do it for teams outside the EPL are just not in place, and can't suddenly be implemented overnight for one-off games like FA cup ties.
The teams you mention may possibly still have some of the equipment left over from when they were in the Premier League (or they may have disposed of it, for instance by selling it to newly-promoted clubs). However even if they do I can pretty much guarantee you that they're not going to be spending the time, effort and especially money to keep it ready for use, when it's not a requirement for the league they're in.