Bronze is a naturally occurring metal alloy (mix) which is stable and unreactive (it doesn't react with water, oxygen etc and so change), so it is found as it is. Iron is a reactive metal (it rusts when exposed to air and water, forming iron oxide) and so is not found in its pure state, but in Iron ores, where it is mixed with other substances. It needs to be reacted in order to separate it from it's ore. Primitive people way not have discovered that, or had the means to extract it.
Because Copper was able to sustain itself for longer meaning it was much easier for people to come across whereas Iron was reactive with water and air, so could not withstand for a long period of time therefore people came across it left often (and generally in the state of Iron oxide, from reacting with the air!) Hope this helps.