Reason is simple, he was worse than this season and as average/decent, whatever you want to call it, as previously in his career. 2016/17 might be his best season yet, but one good season shouldn't be enough for us, especially at his age. I would have no problem to pay that sort of money for him, if he was constantly playing like he was at the World Cup or the Euros, but that's just 3 and 4 games respectively. If United invest a lot of money in a player, there should be either a bigger sample size that a player has consistently played at a high level or the talent should be very obvious. Neither is the case for Perisic IMO.