Why is David's name spelled דויד and דוד ? with a י and without a י?
The reason is because of david's friendship with Jonathan. A yud was added to David's name and a 'hei' to Jonathan's. To show that their great friendship had hashem's blessing. Since these were 'additions' one doesnt use them in spelling their names. Although R Yehonasan Eibeshuts did add the 'hei'.
I will have to look for the source.