The film clearly demonstrates that nukes kill the kaiju. The one that was detonated above the rift, killed the remaining ones, including the cat5 one, unless I remember it wrong.
So why don't they just nuke them as soon as they are detected, one by one?
The walls wouldn't be needed, if they just nuked the kaiju at the rift in the first place.
I just don't think the writers gave it any thought. Simple as that. No need to make up reasons why this or that - it's essentially the magic egg on the Sulaco all over again.