The ever changing lifestyles of humans, that is what has happened to the Dawn Chorus.
We love to illuminate the night sky, the poor things think its still daytime! they don't notice nightfall, and recognise the dawn even less. Also, they have been up all night because they think its daytime, so even if they did notice the dawn arrive, they would be too knackered to sing anyway.
If you don't believe this, pop along to your 24hour supermarket at about 1am, you will see the birds flitting around under the high powered car park lights, just like they do in the afternoon.