Hallo Abhishek,

when you hit morse key, then AM carrier is modulated by some tone, sinus for smallest bandwith. But when morse key is not hit, carrier is switched-off. That's how I always thought, that morse code transmitting is done.
Result is, that, like Arif stated, very small power is needed for reliably transmitting information. Also, dot and dash are very easily distinguishable, which makes transmission also very reliably. And, finally, use of certain wavelength helps to transmit information with small power over very long distances.
I think, that are the reasons why 'old fashioned' morse code today is still so extensively used.

Bye,
Kai