Why can't I sing low notes?
You can't sing that low because your vocal cords aren't built to vibrate freely at that pitch. Often times a younger child or young teenage girl can't sing low because their cords are underdeveloped. As they mature their vocal cords mature also. Then they can hit the lower notes their genetics will allow.
How do you sing low notes?
Your vocal cords have to be relaxed in order to produce low notes. Before you start singing, give yourself 5 minutes or more to warm-up. Make a sighing noise and try to carry it to the lowest, deepest pitch possible. Repeat this same noise repeatedly until you feel relaxed and ready to sing.