The Cloak of Elvenkind requires an action to pull the hood up or down, which gives you the benefits of the item. In the PH page 190 in the side bar it explains examples of things you can do in tandem with your movement and action, one of which is pull the hood of your cloak up and over your head. So than why does the Cloak of Elvenkind require an action?
In the PHB on page 193, under Actions in combat -> Use an object:
You normally interact with an object while doing something else, such as when you draw a sword as part of an attack. When an object requires your action for its use, you take the Use an Object action. This action is also useful when you want to interact with more than one object on your turn.
As to why the Cloak of Elven Kind requires you to do that? Magic.
If you feel the need to rationalize this behavior for the purposes of roleplay, you can imagine the magical effects of the cloak to take a few seconds to set in, much like natural camouflaging animals take a moment to change their appearance.
The reason is game balance. The designers must have felt that activating the cloak on the same combat turn as attacking or casting a spell was too powerful.