Because he's the better footballer.
Oscar is on track to score more goals than Hernandez scored when Hernandez was playing in a better side, and Hernandez scored two penalties. So 18 open play goals in 15/16 for Hernandez, 11 already for Oscar.
Go figure.
On your edit regarding their fees, you can't judge a played by their fee or else Messi signing on a free for PSG means he's worse than Renato Sanches. Utterly nonsensical.