Why would it be hard to find the ideal light intensity if the temperature were very hot or cold?

Remember that CO2, light and heat (to activate the reaction and keep the enzymes happy) are all needed for a reaction. When one goes missing, it destroys the whole process.

1. No light = no reaction can be made. That means the plant wouldn't intake any CO2 anyway.

2. When very hot the enzymes will denature (plant dead) or when cold, the activation energy will not be there. So once again, it will not respond to light as it is not its primary concern.