Well cycling through all 3 made some people find it difficult to chain things up, some people just want to focus on two weapons instead of three. If you wanted to get from two weapons that were far from each other for instance if you wanted to go straight from a move with rebellion to Lucifer you'd double tap the trigger versus when you want to select a weapon of vergil's it's one button switch to the wanted weapon regardless of which one you have equipped.Why not then just make it like the last game? You were able to switch between all your weapons at any given time.
Also that control scheme to make it so that if you had a large number of weapons to switch from one would have difficulty cycling fast enough to get the right timing for a combo. It would be like if you had only one button for style changes in 4 and had to cycle through an unchangable listing of the styles to get to the one you want to use. It would heavily break flow if you were to have too much to cycle through.