Also luck (says someone with a pathetic H index!) But - I don't care about it, so I can make the observation that of my peers in my 20s the ones that went on to get a massive H-index lucked out with an early paper that became widely cited for a random reason. They introduced a definition, repurposed something that they had done into a fashionable domain, or "got adopted" into a paper by a famous professor and got lots of profile for it. Once they got profile subsequent papers attracted many citations and a virtuous cycle kicked in.

Of course all these people are mega smart and do fab research - but they got lucky, and there were lots of others who were as smart and got pushed out.