Jesus drove the merchants and money changers from the Temple because they had turned the sacred space into a marketplace, exploited worshippers and blocked the Court of the Gentiles from its intended purpose as a place of prayer for non-Jews. The temple was "a house of prayer for all nations".
The merchants were taking advantage of pilgrims who couldn't bring their own sacrifices. The money changers we charging unfair rates converting Roman currency into "acceptable temple currency".