Why didn't Kamesenin just chose to destroy the saiyan space ship when Vegeta and Nappa was on their way to earth? During Vegeta Saga

As I know, Bulma has asked Shenlong to kill the saiyans before they reached Earth. But, Shenlong said that the he couldn't do that because the Saiyans are stronger than him.

I am just wondering.. Since saiyans couldn't live in outer space. Why wouldn't they wish for destroying their space ships instead of resurrecting Goku? What's your opinion about this?
TL;DR : God is not strong enough to destroy those ships, therefore Shenron isn't either.

For the same reason why Shenlong can not kill the Saiyans, he may not be able to destroy the ships.

On the Dragon Ball Wiki :


  Shenron can grant a wish within its powers as long as it does not kill, create love, repeat a wish he previously granted, surpasses its creator's power, and a few more restrictions.


We do not know the resistance of those ships, but we know two things


It was built with really higher technology than the one present on earth
It is strong enough  to resist a fall and crash from space


Considering those, we can assume that God, which is as strong as Piccolo at this moment of the manga, is not strong enough to destroy it