I wasn't present myself, interested too about some of the explanations on why Vaum was picked, but I don't think a Mars Bija correctly encompasses the nature of the sacral chakra either.
It has that, but it also has the qualities of Kraum, which are the same as those of Mars.
Sterile, and dry isn't just about that.
Varuna has the qualities of Neptune, which are for many dangerous.
Kraum creates a safer approach, since it is a bija of Mars (a cognizant planet).
Whereas Neptune is transcendental and has a deep "water" personality in relation to mind.
It's certainly a mantra that can be used on the upper chakras with ease.
In the past, and perhaps even today, the Sacral chakra is thought to be of the water element and feminine.
So based on the basic descriptions of the sacral chakra as being feminine in nature, I can understand why the mantra was chosen.
However, Kraum gives various abilities related to virility and willpower.