Why did Bhishma wait till Uttarayana to leave his body?

From: Chethan

What is the reason behind Bhishma waiting for Uttarayana time to leave his body? In BG 8th chapter, Krishna tells about how a soul leaving body at specific time can attain Him, but later tells for My devotee there are no such restrictions. Then Why does Bhishma need to follow these timings? One understanding I have is ,Krishna`s plan. But are there any other ways to see this?