This may explain why.
When using a halogen headlight bulb, a reflector directs and amplifies the light (the bulbs are omnidirectional and the reflector will bounce the light forward). If you replace the halogen bulb with an LED and the LED bulbs you choose don’t have a reflector system or another means of properly directing/focusing the light where you need it, you might not get the best illumination.
Headlight housings are specifically manufactured for the type of OEM bulb the manufacturer plans to use. To do a proper conversion, the reflector must also be changed to match the type of bulb to be used.
Peter