[akira2501]

Your engine is part of your crumple zone. You do not want your battery to be. Same reason the fuel tank is usually in the back of the vehicle.

[djaychela]

Engines do not crumple, and a proper electric conversion will retain appropriate space.

Fuel tanks are not "in the back" for good (see: ford pinto) reasons.

[pvaldes]

Fuel tanks are in the back for solid reasons. But they are placed before the second wheel axis. In case of crash that area will protect the tank. Pinto had it after the rear tires, so totally unprotected.

A big battery in the place of the motor will turn you into a BBQ in case of collision because fire will fall directly towards the driver. Lithium batteries start burning when hit with some force, and can't be stopped once they start burning.