Bollywood movies are many, Bollywood celebrities a little less, so it's obvious if one actor is too busy to do the movie, another actor replaces him but when it comes to the Khans, suddenly this fact of being replaced becomes important enough to make the headlines.
If one filmmaker approaches an actor to do a role, its not necessary that he/ she does it just because they were offered the role. Sometimes the actors are busy, or they don't like the script and sometimes they just don't feel that they fit the bill. Similarly the Khans of Bollywood too have rejected many such movies, regarding which SRK was once quoted saying, " I did movies rejected by Aamir Khan and Salman Khan, and became a star."
True to his words, these stars have rejected quite a few good films which played a very important role in Shahrukh Khan's career. While Salman Khan gave Shahrukh the very big hit Baazigar, Aamir Khan has given yet another cult movie Darr. Even today Shahrukh Khan is remembered for his performance in the movie and his dialogue "K...Kiran" will always be remembered.
The other two Khans seemed to have played a vital role in causing Shahrukh Khan to become the Badshah of Bollywood. When Darr movie was offered to Aamir, he rejected the movie as he didn't want to do a negative while Shahrukh was more than willing to do an anti hero role and the rest is history.
It's not just this movie, there are many such Bollywood movies which Aamir Khan rejected and these movies landed in Shahrukh Khan's lap and became super hit. Check out the iconic movies which Aamir rejected and Shahrukh did.