Because this is the initial configuration also taking into account relation to the VFM5. This is the intended start point for the vehicle as it enters the game. Depending on how it does and feedback on that, then the developers will review the case for new shells should they be called for.
Shells are down to the developers to decide on, that is the point. This has been the same standard since tanks were introduced. They are always decided by the developers based on where they intend to start the vehicle and what role they want it to fill. Right now, the XM-8 is where they intend i