Early electric vehicles were often adapted from combustion models. Almost all serious designs are now built on dedicated platforms, because the two power sources want fundamentally different packaging.
A combustion platform is shaped around its engine
Combustion architecture reserves a large volume at one end for the engine and gearbox, a tunnel through the cabin for the transmission and exhaust, and space for a fuel tank.
None of those volumes suit a battery. A pack is flat, heavy and wants to sit low and spread across the floor, which is precisely where a conversion cannot put it.
Conversions therefore fit smaller packs into the spaces available, which limits capacity and places mass higher and less evenly than the designer would choose.
The flat pack reshapes the whole vehicle
A dedicated platform places the battery as a structural slab between the axles, which lowers the centre of gravity substantially and improves both handling and rollover resistance.
Removing the engine bay allows the cabin to extend forward, so interior space grows without the vehicle growing, and the wheels can be pushed toward the corners.
The pack also contributes stiffness to the body structure, which means it is doing structural work rather than being carried as dead weight.
Thermal management becomes central
Battery life and charging speed both depend on keeping cells within a temperature range, which requires cooling channels designed into the pack from the beginning.
Retrofitting that into a structure designed for a fuel tank is difficult, and inadequate thermal control is a common weakness of converted designs.
Purpose-built packs also integrate cooling with cabin climate control and with preconditioning for charging, which a bolted-in system generally cannot do as effectively.
Electrical architecture is rethought too
Higher system voltages reduce current for a given power, which allows thinner cabling and less heat, but they require components rated accordingly throughout the vehicle.
Dedicated platforms consolidate control functions into fewer computers with faster networks between them, partly because the wiring itself becomes a meaningful mass and cost saving.
The economics only work at volume
Developing a platform is expensive, so it is justified only when enough models will share it. Conversions made sense while volumes were uncertain.
Shared platforms across several models and sometimes several manufacturers spread that cost, which is why platform sharing became common quickly.
The result is that the packaging advantages compound with the cost advantages, and a conversion competes against both at once.