Air India is under Star Alliance(or actually will be) and Star
Alliance mandates that member airlines should be bunched together
preferably in one terminal.
However with the same logic i find it weird that Korean Air a
skyteam(rival alliance) member is going to T2...
And btw the airline doesn't have the choice of choosing the
terminal(CAAS mandate). They just put forward their request and
CAAS vetoes it. Usually T1 to T2 are usually not approved. Only
rare cases like this are approved. And T3 has no more spare
capacity. Even SQ can't move their entire operations to T3. SQ will
move to T3 if it could accomodate but T3 can not fit SQ's fleet,
thus the split between T2 and T3. A big compromise, I shall
say...