Thanos is a believer in “the weak perish and the strong survive,” and like any good parent he wants to make sure his children live up to what he believes in. So when Gamora and Nebula were growing up, Thanos had them fight each other. The loser would be taken away and given a bionic upgrade to give them better odds in the next match.
For Nebula it didn’t work. She explains that she kept losing to Gamora. Again, and again, and again, and again. No matter how many times the two fought, Gamora remains largely implant-free
Nebula however is mostly a cyborg by the time we meet her.. Nebula lost against Gamora so many times that she has enough cybernetics for her body to restore itself from a mangled heap after getting hit in the everywhere by an oncoming spaceship.
When a dad is an advocate of Survival of the Fittest and has two daughters - one who has won every single fight she has ever been in and one who needed most of her body replaced with machine parts to keep up - which one do you think he might be most proud of?Gamora of course and theres your answer