Save the airline money!
Catering costs are far higher for day flights than night flights -and the crew have to work far harder. So if they can get passengers to sleep it's better for the airline and the crew.
I book day flights because I hate flying and can't sleep on planes. So I read, watch movies, play games, eat and drink. And I refuse to do so in the dark. If that inconveniences the crew, so be it. The airlines sell their services in their ads, including emails that they send to me - I choose to believe them.
I get really fed up with airlines promising 'cuisine, fine wines, on board entertainment' etc. and then expecting you to get on board and go to sleep!
If daylight is inconvenient for passengers, frankly speaking they should book night flights if it's a nap they're after. Or carry a sleeping mask on board. You don't close curtains on a bus if someone chooses to nap, do you?