Why he waited
The Gita and the Upanishads say that those who die in the bright half of the month and in the six months of Uttarayana go to the higher worlds and do not return. Bhishma, who could die when he chose, would not leave his body while the Sun was moving south. So he lay on the field, teaching Yudhishthira dharma and the thousand names of Vishnu, until the Sun turned north, and died on the Ashtami of Magha.
Uttarayana then and now
Uttarayana truly begins at the winter solstice, around 21 December, when the Sun's northward movement starts. In the age of the Mahabharata the solstice fell close to the month of Magha, so the Sun's turning and Bhishma's Ashtami came together. Because the Earth's axis slowly wobbles (precession), the solstice has since drifted back against the stars by more than three weeks, while Makar Sankranti, the Sun's entry into the sidereal sign of Capricorn, now falls around 14 January.
So today the solstice, Makar Sankranti and Bhishma Ashtami fall on three different days. Many still speak of Makar Sankranti as the start of Uttarayana by custom, while the festival of Bhishma's death keeps its lunar date in Magha.