UEFA have insisted on a clear two weeks at Wembley prior to the CL Final, so the FA Cup final has had to be bought forward and thus clash with a full set of Prem fixtures (including the prospect of the title being won 20 minutes before the teams come out at Wembley).
The preparations for the CL final are not that involved and I believe the motivation is to increase the importance of the CL over the FA Cup, hence the previous move to Saturday.
The FA Cup final is still the most watched football match throughout the world and UEFA are aiming to change that, sadly, the FA are helping.