Why it is very difficult to transform D.C current into A.C current but easy to transform A.C into D.C current?

DC is hard to step down without losses. With AC you can use transformers. When you generate DC you can only transmit it 20 to 30 miles before the resistance of the wire starts to degrade it too much. You start out with 120 volts and at the end you only have 70 or 80. With AC you start out with 15,000 and even after several hundred miles you still have 14,500 volts. Using transformers along the way you convert the high voltage to 120 volts ac. so you can build generating stations far apart rather than every 50 miles or so.

In the beginning there wasn't an easy way to convert dc to ac as there is now. So the losses would be much more than they are today.