How did early renaissance artists learn to create more realistic art than their predecessors

Renaissance artists just reacted to a fashion change. Artists before the renaissance had the capability, technically and from a material point of view, to create the same art as during the renaissance and following periods. The main difference is that the pre-renaissance art was still based on standard iconography, as e.g. is still the case in the orthodox churches. During the renaissance fashion changed in favour of more realistic, although also idealised, figures by copying the old greek and roman sculptures.