The explanation why Our Lord drove away the money changers from the Temple we can find reading the Gospel text, but who the money changers were and what they did in the Temple? Here is the answer I have found:

The money changers in the Temple were there because of the Roman occupation and the insistence that Jews pay for their sacrifices in shekels. The Romans only allowed Roman currency in their conquered nations, so if you wanted to make a sacrifice in the Temple you had to exchange your Roman currency for Temple currency. The money changers provided this service in the Temple courtyard, but they charged a hefty fee for doing so. They made quite a lot of money doing this as Jews were required to make sacrifices frequently, and your sacrifice was either money, which had to be in shekels, or a sacrifical animal, which had to be purchased right there in the Temple and also purchased with shekels. While the high priests did exchange back sacrificed shekels for Roman currency (to purchase what they needed in the markets), the wealth in the vaults functioned as a kind of Jewish bank. You could obtain a loan in Roman currency from the Temple vaults at very high rates of interest. The Gospels use the phrases "den of thieves" and "house of merchandise". The first is obvious on its face, as the money changers were stealing money from people who were just trying to follow their religion. The latter was a reference to the sacrificial animals available for purchase in the Temple.

Lk 19:45-49

And entering into the temple, he began to cast out them that sold therein and them that bought. Saying to them: It is written: My house is the house of prayer. But you have made it a den of thieves. And he was teaching daily in the temple. And the chief priests and the scribes and the rulers of the people sought to destroy him.

Picture by Alexander Bida 'Jesus drives away the money changers from the Temple