Why does my steel level remain much lower than other resources in this game?

Iron is the slowest to produce of all the resources. Food and fuel are the fastest, and timber is halfway between them. This also affects the amount of resources you pull out of other cities when robbing. Assuming there is a large amount of everything, you will collect them in a ratio of

2 : 1.5 : 1 The only time this distribution will be different will be if there is not enough food or fuel or timber to fill that section, at which point the remainder will be redistributed through the others.