Continuity mistake: Susan crushes a chair under her foot when she first meets the rest of the monsters. But moments later, the crushed chair by the table is gone, with two perfectly good chairs right next to it.
Other mistake: There is a shot of a random newspaper blowing around in the middle of San Francisco. But the paper is from New York, with headlines about New York.
Continuity mistake: The man running up to the door for the joke about having to have so many things scanned, shows separate scanners being used for his hand, both elbows, tongue, butt, etc. When he's running up to the door, it shows there is only one scanner. But there are multiple in the same spot as the door opens for him.
Chosen answer: Well, it's a fiction movie; some suspension of disbelief is always needed. There could be a number of explanations: for instance, lifeforms such as insects on the grass, bacteria inside them or even grass itself, being unable to absorb quantonium. It might affect only sentient beings. Or Susan might even be the first being to get in contact with the substance - this would be possible depending on the shape of the meteorite container, the exact means by which quantonium touches her body - which isn't shown... but, first and foremost, this happens because its not happening would leave the filmmakers with a plot issue to be dealt with - so, other beings don't grow simply because they don't.