Airlines can and do fly at other times as well. Most of my knowledge here is second hand (from other FT posts, net wisdom gained over the past few years and 75+ one way trips to India). QF used to arrive at a decent hr into BOM (discontinued for now). Canada3000 (??) was planning on arriving in the middle of the day into DEL via polar route and depart in late afternoon. Also some of the flts to Gulf states fly during day time hours.

Reasons for night arrivals/dept seem to be:
- There are concerns about heat and fuel loads (so most of the time they take off and land at night).
- Arriving into hub cities in Europe to hit the first bank of flts (since destination is dispersed hub cities such as LHR, FRA, AMS, CDG do spread the traffic that originates in BOM and DEL to a lot of destinations in the US and Europe).
- Concerns about fog (particularly in Delhi but also true in Bangalore, Bombay, etc.)
- ATC issues (domestic traffic drops off at night).
- Customs and immigration staffing efficiency (the govt might work with you if you bring a lot of traffic but otherwise, you will get skeleton staff).
- Most of the commercial flts carry freight traffic as well and their cut-off time is usually 2 to 4 hrs before departure. Night departures allow air shippers to ship today's orders.