two reasons:
1) fewer vehicles in a platoon (and similar restructuring elsewhere and upward) creates a division with considerably less vehicles which makes for a more agile division capable of more rapid deployment and redeployment. It also reduces the logistics burden on the division. The rationale that more of everything is always better is not necessarily true.
The platoon is not significantly weakened by losing a vehicle. With four LAV ISC you get at most 28 dismounts, no CUAS capability and no indirect fire capability. Restructured with only one ISC and two non turreted versions you have 25 dismounts plus an AD/CUAS component and a medium loitering munitions carrier.
2) a higher ratio of dismounts to people remaining in vehicles creates greater combat power on the ground. Vehicles that stay far enough forward to provide fire support become magnets for unpleasant fire. It is better to support dismounted troops with indirect fire, whether their own or through battalion mortars and artillery systems. The vehicles are better off to stay back in dead ground or whatever cover is available, from which they can provide air defence and indirect fire support (such as through loitering munitions)
The point here is that the LAV is not an IFV. It's an infantry transport vehicle for infantry that fights, or ought to fight, dismounted. The LAV ISC loses too much dismount space to the turret and keeps three people with the vehicle. A LAV without the turret can hold a crew of two and up to 9 dismounts.
Or so the theory goes.
