Because it's only obvious in hindsight, and was he a player Ange wanted? Probably 10 players 2 years ago that were on the brink of breakthrough, where are they now?
We can afford 70M, if he was the right guy in summer, do it early, do it at the price asked or even more (United overpay for everyone but still manage to get players while with a brick manager and not in Europe, and yes, I get they are United).
Saudi Sportswashing Machine, City, Liverpool, Scum, Chelsea have all gone out for that 70M+-100M+ player in last couple of years, we seem stuck in the 60M range for our top players, we can't compete all the time but we badly need 1 or 2 of those, we can rely on the Xavi/Bergvall/Odobert/Gray signings with the hope they all come good