Hello,

why do elephants with bow not have shoot and charge ?

one would think that being shooting platforms, even more stable than chariots, they would get shoot and charge.
why would the crew stop shooting as the elephant charges?

Probably because the Bow ability for nellies doesn't represent their crew but supporting infantry acting in close cooperation with them. Additionally I would not be surprised if nellies aren't representing fewer individuals than chariotry - although this is not actually stated anywhere.