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

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.

Carry-on luggage is generally broken into two categories - "Cabin bags", and "Personal items".

There's no strict definition of these terms even within a single airline, let alone across the industry, but in general a "cabin bag" is a larger item that would go in the overhead bins, whilst a "personal item" is a smaller item that would go under the seat in front of you.

Most airlines generally allow (economy) passengers to bring one of each item - one bag plus one personal item, with the expectation that the larger bag would go into the overhead locker, and the smaller item under the seat.

By restricting carry-on to a single personal item Turkish Airlines is likely looking to achieve a few different things.

1. Speeding up and simplifying the boarding process. Presuming most people put their personal item under the seat in front of them, then this will be much faster than having to find space in the overhead lockers and put one item there, then subsequently still have to carry a second item to their seat. The simple fact people are carrying less as they board the plane will also make things a little faster and likely reduce much of the friction that occurs during boarding as passengers try and get past others who are busy stowing their bags.

2. Reduce people leaving their seats during the flight to get something from their bags in the overhead. With fewer bags in the overheads, there will be less need for people to stand up and access them (which often also requires others to get out of their seats).

3. Speeding up and simplifying disembarkation. Similar to boarding, less time will be wasted with people finding and reclaiming their bags from the overheads.

All of these will help reduce the number of people that are coming in close contact with others to at least some extent. Whether it's truly enough to make a difference is something that most likely can't be shown one way or another.

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.