Well, there is no particular 'reason' for this. Maybe, at the time, it seemed like a good idea, more than 10 years ago. To be honest, only GameFreak can answer this satisfactorily, and I highly doubt they have an advanced reason such as legal issues or coding issues or so on.
But, one thing pops up to the mind in my opinion. One of the Pokemon to gain the newly introduced steel type in Gen II was Magnemite and its evolution, Magneton. These Pokemon are based on magnets, which are closely associated with steel, and others metals are you said were too generic, i.e., not specific. Thus, to show the 'magnet' part of it, it may have been named Steel (although Iron seems better, but 'refined' or 'advanced' or 'processed' iron is steel, which may have won out due to its superiority over iron in physical properties.)
Once again, this is merely my opinion in this matter.
