Why did Turkish Airlines restrict hand luggage to 4 kg for the duration of the COVID-19 pandemic?

The ideal is that each passenger on boarding goes straight to their seat and immediately stows their carry-on near their seat. At the end of the flight when it is their turn to leave, they simply stand up, pick up their carry-on, and walk to the door.

In the real world, there are problems like early boarding passengers sitting near the back of the plane stowing bags in bins near the front in case there is no space near their seat. Passengers near the front then take their bags back through the plane until they find a space. During disembarkation they have to work their way back against the flow to pick up their bags.

I don't know Turkish Airlines reasoning, but ensuring that what bags are allowed on board will fit quickly and easily near the passenger's seat seems likely to lead to less movement around the plane and contact between passengers.

How does reducing the amount of hand luggage reduce movement around the cabin

This would seem to be trivially answered:

On flights, overhead items are inevitably "a mile away" from the item owner.

Resulting in the usual endless walking around of pax to get items; pax standing over other unrelated pax "miles away" from their own seat while fooling with their items, etc.

Secondly, during boarding / departure there is a huge amount of disordely "pushing around against each other" while pax stuff their usually enormous bags in to overhead. Inevitably, other pax or staff have to help people with huge bags, inevitably enormous bags brush against everyone, people fall over in to other people, etc.

Obviously, eliminating cabin baggage (or slashing the amount of cabin baggage, as they have done) would end all this.

A user above in a comment has pointed out that flights are usually not nowadays full. Maybe, maybe not. A few flights are still very full due to cancellations and so on. Setting aside the "flights are not full" thought, the reason for the restriction is completely obvious. It eliminates the usual endless "milling around" caused by cabin baggage.