The answer for your question is very simple, they didn't sell those Bitcoins they bought too early maybe because they are not yet satisfied with Bitcoin's price, maybe they were waiting for this very right moment, because selling all of their Bitcoins now is surely a big profit. Luckily he still find his paper wallet, because that's the most important thing to your treasure. I assume that this guy was busy with something else in his life that's why he forgot about his Bitcoin until now.
Don't think so, these people who bought bitcoin back then did not know that it will come this far, if they knew then they would have taken care of their bitcoin and the address as to prevent it from being lost. I am happy that the person still has the wallet, but I don't think that it is being busy that caused that person to lost track of the bitcoin that the person has bought because bitcoin is not a big deal for many back then, maybe that person just jump in and bought some then forget about it.