Because anyone who chucks a (copper-based) rock into a standard wood fire will notice the copper that melts out of the rock. Tin's melting point is only 500F for example (one of the components of Bronze), while Iron's melting point is ~3000F.
It has to be the _correct_ rock, but chucking random things into a fire is an activity that all humans like to do. Its only a matter of time before some human chucks the right rock into a wood fire to discover Tin and begins the Bronze Age.
Iron however, is far more difficult to process. One theory is that early Iron-age was helped by meteors, as pure-iron rocks from the sky would be objects of worship to any human. After experimentation with the space-metal, they'd discover the incredible properties of iron.
With the "Seed of knowledge" planted in the human's brains from space rocks, humans would build hotter fires that can better process the space-rock / space-iron (maybe not reaching 3000F, the melting point, but maybe 2500F for hot enough to make it malleable / hammerable).
Once this discovery is made, its only a matter of time before people chuck _other_ rocks into the 2500F fire and start hammering to look for the common iron that's all around us. So the process is instead to heat the iron as hot as possible with your current technology (again: maybe 2500F) Then, you smack the hot rock with a hammer until you squeeze all the impurities out (while shoving it back into the fire to keep it at 2500F+ as much as possible through this process).
Once the impurities are gone, you're left with pure iron, equivalent to the space rock.
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This iron-age process (even for the simplest of ores) is far more difficult to "discover" than "chuck the correct rock into a fire".
You really can start mass producing iron when you can melt the darn thing, which requires a blast furnace. But "Iron Age" smiths were largely with simpler forges / fires and with the "hammer method" IIRC.