I suspect its because an injected engine will continuously squirt fuel into the inlet when the mixture is rich, throttle slightly open and the pump is on. This happens whether the engine is cranking or not. Therefore this runs the risk of creating a big puddle of fuel and a fire risk.
Carburetted engines do not do this as the fuel is only drawn in when the engine is sucking air in.
Both types of engine are primed in a similar way, the carburetted engine has a manual primer to squirt a bit of fuel. With injected engines you switch the injection system on far a few seconds to achieve the same thing.