This is what the FA say
Prior to the 2024/25 season, the FA Cup employed VAR in matches staged at Premier League home grounds, as well as the semi-finals and final at Wembley Stadium. Clubs outside of the top tier - even those with VAR capabilities that had dropped into the Championship - went without the technology in home matches.
However, to ensure fairness and consistency across the competition, VAR has now been scrapped for the fourth round and prior. Even Premier League stadiums did not employ the officiating system in the third round of this year's competition.
The decision was made to make sure that the same rules were applied for every team across the competition, addressing previous complaints about the sporadic use of VAR and how it created an imbalance between top division and lower league clubs.