Maybe because is not stable enough and having stability is the most important thing for a player while Lukaku doesn't have this. Also, Lukaku is not a cheap player and Inter Milan should spend too much for it while he is getting old, and currently, he is 30y old and not young anymore.
I think seeing his performance last season of course Inter Milan have to think twice in making this decision where they need a stronger competerior for Martinez on the front lines And Lukaku is the best tandem that can be reunited next season. I don't think it's a big gamble if Inter Milan does the option to make Lukaku permanent as their player after his loan period ends. One of the Inter Milan strikers is confirmed to have left Inter Milan. He is Dzeko, who is reported to have found an agreement to join Fenerbahce, so Inter Milan must think quickly to find a replacement, and Lukaku is a player who was quite integrated into their playing scheme last season. I don't think 30 years old is considered old for Lukaku because he has a body that is quite stocky and very physically strong so that he can be relied on by Inter Milan next season. I think Inter Milan should buy him from Chelsea otherwise they will have big problems up front line next season. I think Inter Milan is a strong candidate team to win the Scudetto title next season, and they have to understand that, namely to make big changes in this season transfer window.