When Mani Ratnam approached Shah Rukh, he was quite interested. However, when Mani Ratnam reached Mumbai, SRK was already committed to Aditya Chopra's Rab Ne Bana Di Jodi,making his schedule tighter. Mani Ratnam did not want to delay the project, hence why he decided to cast in AB Jr. instead!
After Chalte Chalte, this is the second time we won't get to see the SRK-Aish jodi! Tough luck for all SRK-Aish fans!