cause he was young, promising, in very good form, and he was supposed to have a decent resale value if things go south (which is the case right now)
as for lukaku he's 30, missed like 20 games last season due to injury, dzeko benches him, he's fucking expensive, not to mention his salary, personality, history with juve, and he's an especially dumb mofo in a profession that is not very famous for the bright minds of its professionals
yes, he's an effective striker. and the list of positives end here. he's the request of a coach who shouldn't be here, was in his best form under conte (god save us from his return), failed at chelsea and manure, is a known choker, technically he's not even better than vlahovic, has zero resale value, etc. for me it makes literally zero sense