While playing Pokemon GO, you may have noticed that some Pokemon types are much easier to come by than others. For instance, Fire-types seem to be rarer even though you can catch them just about anywhere (even near water). Another of those rarer types is Steel Pokemon, of which there are only two in the game right now: Magnemite and Magneton.
Like most other types, Steel Pokemon seem to stick to very specific geographical locations, namely those that have a lot of industry around, or, at the very least, a lot of buildings. Areas with factories, warehouses, or just lots of structures (like New York City) seem to have a higher focus of these types in Pokemon GO than other areas.
While you can in fact capture a steel type outside of one of these areas, they are exceedingly rare. Parks seem to be devoid of these types, as are water areas not near factories. Your best bet is to stick to large concentrations of buildings and industry.