Due to the fact that all major cruise eships are built in countries that have 220v power supplies, the ships run on the same power specs.
It is only for the electrically challenged people in North America that they offer limited 120v power in passenger cabins - via large, expensive, heat generating step-down transformers that are installed in the void spaces betwen the walls of passsenger cabins.
For the rest of us; we either live in countries that have 220v appliances, or we have multi-voltage appliances that will work with nearly any power source.
Why don't they beef up the 120v power systems on ships to accommodate increased demand?
COST. Those transformers are very expensive. Larger ones are far more expensive. Larger ones generate a great deal of heat and can become fire hazards themselves.
SAFETY. With the situation today, anyone dumb enough to bring high wattage devices that can cause fires will trip the circuit breakers and identify themselves. There are many, many dumb people doing this these days. The fragile electrical system becomes an additional fire prevention tool.
A ship's electrical system works rather differently than a system on land.
There is no real "Ground" to prevent electrocution. That's why the electrical outlet in your cabin bathroom is so limited. If you could plug a high wattage device like a hair dryer into that outlet and then drop it into the handwashing sink or bathtub, you would be dead. Since most of the ship is made of metal, electrical hazards are far more numerous.
For decades, crew have been forbidden to dring ANY electrical equipment aboard (including extension cords) unless it has first been tested by the ship's electricians, to ensure that it will be totally compatible with the ship's system.
Trying to do that for 3,000 - 5,000 passengers every week would be impossible. A limited electrical system does a far better job of screening for the idiots. A surprising number of geniuses bring their electric hibachis on a cruise.
FYI - the 220v power sockets in the cabins are NOT running through the step-down transformers. They can handle a very big power load with no problems.
Plugging a multi-socket block into just one allows you to plug multiple 220v (or multi-voltage) devices into one outlet with no problems.
Note that this could potentially be a fire hazard, as the sensitive breaker switches on the step down transformers that inhibit the 120v idiots will not stop the 220v idiots.