Age
Mandzukic is in his 30’s and whilst that isn’t massively old for a footballer the Hammers will only get the best out of him for a few years at most. After that they won’t be able to sell him on and he’s more than likely to boast significant wages so financially this deal has issues. The Hammers moved to the London Stadium to highlight their commitment to the future so signing a 30 something forward wouldn’t make sense with their policy.