Why in some countries 110 volts AC is standard electric supply? ?

A technical reason is the cost of wiring is lower with the higher voltage, because you need half the current for the same power. That means the wire can be thinner, so cheaper, but only marginally so in practice.This leads to lower voltage drop, so everything works better.

The safety of 110/120V may be perceived by some to be better but you are just as dead whichever, and you would be stupid to grab either in your hands. I have seen people lick their fingers to touch 240V to see if it is alive. It depends how dry your hands are, and how calloused (work hardened) the skin is. The accepted "safe" voltage for AC is 32V, though under particular circumstance that would not be safe either. Voltages below 600V are usually classified as low voltage because the characteristics are similar. Below 40V is classified as extra low voltage. In other words, the same design and safety precautions are necessary for 120 or 240V systems. The noticeable difference is that the wall outlets are 20A instead of 10A or whatever. I think the US has long gone away from 2 pin wall outlets, but no doubt they are found here and there.

The reason 110V was chosen (and it all comes from US influence) was because it suited the light bulbs of Edison, who certainly wasn't the first to distribute power by a long shot, even in the US. He had a louder voice and bigger ego, and certainly got to be influential. It was only a few years before everyone figured how to make higher voltage bulbs (double coiled filaments?). The lethargy towards change? Well most households in the US apparently have 120 and 240, so there is the higher voltage too so that appliances like clothes driers and electric stoves can be used, so there is no real need to change anything. The analogy with metric changeover is a good one. I read somewhere (Scientific American) it has been the legal US standard since about 1850 (cannot remember the exact date). You could say that the US because of its size can hold back on these issues. I am sure the arguments both ways are mostly emotional, or about national pride or something.

The link someone put in talking about unreliable old European infrastructure is incorrect and silly. The US has huge blackouts covering whole regions just like or maybe more than anywhere else. However this has little to do with 110V or any relative age of infrastructure. The US distributes power at much higher voltages like everywhere else, because that is technically necessary. The US also has much bigger systems, so larger blackouts are more likely, so it not about a failing of any particular system.

When electricity was 'the new thing', Europe and other countries on the other side of the Atlantic rushed into it and created their electric infrastructure a bit prematurely as far as realizing how much energy it takes to power modern electronics. They didn't look far enough ahead and my guess is they over estimated. So their grid was built on the range 210-240 Volts (AC) at 50Hz. (Voltages can actually fluctuate which most electronics can handle).

Now for the U.S. The States gave a bit more planning into how they would setup their electronic grid and so it actually became available (commercially widespread, that is) after Europe. Since this is the home of Thomas Edison, we had the genius who said that the future will only need 110-125 Volts AC at 60Hz (this is a faster rate than Europe's). But the U.S. did not stop there. Have you ever looked at a pylon which runs 3 or 4 electric wires on the tower? The bottom wire closest to the ground actually serves the area with power and runs at 110 Volts. However, the higher up the wire in the stack, the more voltage it is actually carrying, even up to 380,000 volts. Pretty smart for widespread distribution.

To put it all simply, it depends on the infrastructure and how to transport energy long distances and cut it down to a usable resource.

When Edison designed his system he chose 110 volts and that is what you get on average around the country. Appliances are designed to run on a range of voltages from 105-120 volts. When direct current fell out of favor (because it couldn't be sent long distances as easily as alternating current) they adjusted the AC voltage to mimic the "power" of 110 volts DC. Since AC current is a sine wave, the voltage coming through your house actually varies from +170 volts to -170 volts with the average power the effective equivalent of 110 volts DC.

Countries using 220 volts electric power supply are more concerned on economics than safety. Using 220 v require smaller conductor sizes but at a higher (more dangerous) voltage. While countries using 110 v system are more concerned on safety than economics. 110 v system requires larger conductor sizes but a lower and safer voltage.

110 is believed to be less dangerous. This is the main reason. We're at the point that no one wants to change . Kind of like we should change to the metric system but you see how well that is going lol