The Moon is the mind.
The mind evaluates things as either “positive” or “negative.”
This causes the mind to have desires for the positive stuff and against the negative stuff.
These desires cause emotions, which boil down to: happiness when we acquire the positive stuff and avoid the negative stuff, and unhappiness when we can’t.
It’s the “desires” part of the mind that makes the Moon a vaiṣya planet – a planet indicating finances. Desires inspire us to earn money (etc) so we can attain the stuff we want and get rid of the problems we don’t want. In other words, earning money is a means to fulfill desire, and therefore the moon is crucially involved in how well one earns money.
(The other vaiṣya planet is Mercury, because it governs our ability to exchange, which includes trade and commerce.)