To be fair, when Otamendi was 25, he had gone unbeaten in the league and win the Europa League with AVB's all conquering Porto side, it's not as if he's come out of nowhere. He's been highly rated and linked with everyone in pretty much ever window for the past 4 or 5 years, I'm not sure why he never got that big money move. One game at international level is really not a great way of judging a player either, given that so many players play completely differently in internationals and he wasn't used to playing with those players. He was the best CB in la liga last year, which means a hell of a lot more than one ropey game for an Argentina side that seems hell bent on making everyone look worse than they actually are.
I can only assume that people just weren't willing to take the gamble on him, possibly because of his height. Valencia took the risk and he showed everyone that he is the real deal, that's why everyone is now interested. He's proven himself and those who had doubts now see him as one the best available CB's in world football, of which there aren't a whole lot of options.
With all that being said, we still don't need him. If we needed an experienced, top quality CB then he would be right at the top of my list, but we don't.