I think it's because they know that parts of the game rely, and always have done, on a flexible interpretation of the rules. But VAR removes any flexibility from the rules, it's black or white. If you actually enforce the rule that the keeper has to keep both feet on his line (and they tried to do it about 10 years ago), it basically makes it almost impossible for the keeper to save it. So you give them a bit of flexibility to step a bit forward in the process of diving, but pull them up if they take the piss. VAR ruins your ability to do that because as soon as people look at the replay, it's clear the keeper is off his line, and ask why it wasn't pulled up. So instead they just say that it's not going to be used for that purpose.
But with handball, we've got the opposite problem of a fairly nuanced area being shoehorned into a binary interpretation. It seems to me that some 'confusion' around the handball rule has been manufactured in the past year or so to justify this rule change where intent has been removed from the handball rule. And the reason for that is that intent can't be judged by VAR, without the referee having to go over and rewatch the footage. So it's easier to just say that any handball in the lead up to a goal will disallow the goal, no matter how innocuous and accidental it is, and any handball potentially preventing a goal leads to a penalty. Basically, we've got a situation where instead of VAR being used to enforce the existing rules, the rules are being changed to fit in with VAR. I have absolutely no problem with VAR in principle, but like the offside call the other day, now they've got it, they're trying to create this situation where it's some all-knowing, infallible piece of technology instead of admitting that there can still be doubts. The handball rule should have remained the same with VAR only overturning clear and obvious mistakes like they supposedly do with penalties, and like they should do with offsides.