A2QFI,
I can give you the official version and and the alternative interpretation.
The official theory is that less hand baggage there is, the faster people will be able to clear security, because the more indepth searches will be conducted on passengers and they carry-on. So given that fractional units of hand baggage cannot be introduced, they come up with the bright idea of shrinking a size that has been standard for years.
The alternative interpretation is that this is something the airlines, airport and security wanted for a long times and for different reasons. As you know, basically, the low cost passenger is seen as an inconvenience to everyone, and evermore his baggage, cabin or hold that it is.
Security wants to have less handbaggage to have less stuff to screen, save on machines and operators.
The Airline wants to have less confusion at boarding when people brings all that stuff and some may even ask (God forbids) a CC to place a 10 Kg roller in an holder. Before someone jumps on me, I never asked for anything in my life excpet the time maybe. And less weight is less fuel, of course.
There are even some passengers that are bothered by everybody else baggage and this give the theory another supporting view.
Now for the Easyjet story. They are somewhat smarter than the rest of the other. They understood that posing strict limits on the weight of carry-on baggage will only generate countless discussions at the check-in counters and at the gates, while the bigger part of labour and weight comes from the shipped baggage anyway. So they were trying to streamline operations by shifting at least a portion of the weight from hold to cabin, where is the passenger that does the work. Less hold baggage, faster turnaround,less weight, less claims, more profit.
Unfortunately this smart Easyjet policy is going against the mainstream now, while the traditional line of airline thinking get an advantage by the new measures.