Al Ahli, playing in the Saudi Professional League was interested in bringing Osimhen to Saudi Arabia. The offer was around €150 million for 4 seasons. But Osimhen seems to have decided against it and stayed with Galatasaray. Now it is personal choice and we need to respect it. I am not sure why Osimhen turned down that offer, but he may be having his own reasons.
To be fair, I read that it was about the time, if it was just one year contract or a loan like Galatasaray, then he would have probably accepted that without a doubt, he is still young and wants to play at the top leagues, he can literally be striker for United and score bunch of goals at premier league at this age, why would he want to go to Saudi league now.
It is not that he preferred Galatasaray over Al Ahli, he probably doesn't care about which team he went there for just one season, all he required was to have one season away from Napoli, and then move to a bigger club, that's his intention was and that's what he will get. Al Ahli lost their chance to get him because they offered a four year contract, dude is 25 years old, he doesn't want to play there until 29, he just wants one year.