Why were people born on this planet and not on another?

this is the only livible planet

Well that is simple. Earth is the only Planet known to be able to sustain human life. The other planets are literally inhabitable with the bad conditions. Earth is able to sustain human life and activity with Oxygen and gravity and everything else a person needs to live on. Let's just use a quick "example". When astronauts go into space they can't go without their special/fancy high-tech space suits. The reason for that is because everything in otter space (with the exception of Earth of course) has no oxygen so with out the suits they would not be able to breath. And the temperatures in space are like insane and just like no human could withstand those conditions. If a person went to space without a suit 10000000% guarantee they would not survive.  

i think its the outcome of people's preferences.

which time period is this? or is this story fictional, i'm going to need more info to answer this question