Difference between bronze divisions is too large while difference between gold divisions is too small.
Adding Iron allows them to take the 5 bronze divisions and make 8 iron/bronze divisions, without turning every other tier into 8. I guess they figured that every other tier can be spaced correctly with only 4 divisions each. So they're basically saying they've identified a vast range of players below silver who need more divisions to separate them.
I imagine it looks like this shitty MSpaint thing I made:
https://puu.sh/C150Z/6380831b83.png
I didn't include plat+ because too lazy