He wanted to be popular with the regular, poor people. He was throwing money left and right but always denied where his wealth came from. He wanted to come across as a self-made, honest man. Some believed it, or just weren't willing to accept the atrocities he was capable of. He was overthrowing the government, blaming it for everything that was bad in Colombia, and wanted to appear as a savior, hence the contrast he was creating by wearing simple outfits in certain public appearances. But he did also wear suits and shirts that were considered tasteful and fashionable at the time.