Good question which I asked myself in a recent project too. Here are my thoughts:
If I didn't know these guidelines i would place the action buttons on the right (at least in an environment where reading goes from left to right). The reason is that user tend to scann a card from top left to bottom right.
I do not now the reason why google places the buttons on the right.But if using material design patterns is a condition, i would go with the buttons on the left. The reason is, that be following this rules, every app in material design would work the same way and users will be used to that.
This are only my thought and not the answer to your question on the 'why' but this can only be answered by google.