Question

if about three fourth of the Earth is covered with water why doesn't the water falls on the another planet

Answer 1

Because of gravity the water doesn't fall on other planet

Answer 2

earth is perfectly placed in the solar system that it promotes h20 stability but optimum temperature of not too cold and neither too hot.H20 perfectly needs this situation and also earth's temperature gives rise to the atmosphere formation to prevent the water from dehydration. But mars don't posses such atmosphere because of temperature and gravity also plays a great role in water availability.....however in 2024 elon musk plans in to launch falcon heavy with 80000 people mission to mars in the next decade.
he also made a plan reduce CO2 by nuking poles of mars with two heavy nuclear bombs.

And cost to travel mars is 6crore rs. If we plant trees on mars we can definitely colonise mars and water may be flowing through the martian soil.