Yes, there is an explanation. The explanation is that to be left, and to have something left, simply do not mean the same thing. To be left is the passive form of to leave, while to have something left is "to have" + an object + the adjective "left".
Imagine there has been an earthquake. John's house is destroyed, while Tom's house is undamaged.
We can say that Tom's house was left standing by the earthquake, or The earthquake left Tom's house standing.
We can also say Tom lost little, he still has his house left, but Johnlost everything and has nothing left.
Little was left by the tsunami is merely the passive form of The tsunami left little; the verb here is "to leave" and not "to have", and "left" functions as a participle, and not as a predicate adjective.
Yes, the subject of the verb leave / left is the person or other agency that causes the change; the object of the verb leave / left is the thing that remains after the change. I left my bag in the shop - My bag was left in the shop. The tsunami left very little of the town - Very little was left of the town.