I knew there was something else that caused Spalletti's dismissal as Napoli's head coach. If the Napoli owner always wants to decide how Napoli players will be used, he will make the club not to move forward because no good coach will last as long as the Napoli head coach can't decide which player is okay for him to use in a match. For Conte to identify Lukaka as the number 9, he wants to secure his signature for Napoli, which means that Osimhen can't fit in with his playing style. Osimhen might not have much playing time under Conte, so there's a need for him to push for a deal with another club where he will be the number-one striker.
I would say Osimhen is obvious that he will leave, because he worths a lot of money and Napoli didn't do that well this year on the league, and nowhere else to be fair, which means that they are going to need some money and selling Osimhen would be a lot of money plus Lukaku could be bought for free, just his salary.
I think that is the reason, not preference, if you offer Conte to play Osimhen then he would do that, he would fit any system better than Lukaku for sure, Lukaku isn't the player he used to be so I do not expect much from him. But, if you sell Osimhen then you do not have a choice to get someone, and for that amount of money the best player you can get will be Lukaku, that is why I believe Conte wants to get him.