He was chosen because he performed better than his rival, but there were contributing factors:
1.) Ron Weasley had increased confidence due to the false belief that Harry Potter had spiked his drink with a luck potion. Ron had the talent inside of him naturally, but simply had lacked the confidence to show it. This ruse by Harry, helped Ron shine.
2.) Hermione Granger used a spell during the tryouts to make Ron Weasley's rival make a mistake. The skill levels between Ron and his rival were so close, that Hermione wasn't sure who would be chosen, so she used the spell to tip the scales in Ron's favour.