That was another point that striked me. Not that 777-200 has better capability to fly into fog but its possible that they may want to avoid disruption to the flights from FRA to ORD. Having a flight terminating at FRA from DEL might confine the impact to DEL passengers only. But then the whole thing was managed is quite confusing. A direct switch between AMD and DEL flights would have done the trick.

And not sure about all the connecting PAX. They are using DEL as hub and most of the passengers arriving to DEL on AI 126 today seem to be transit passengers.