It's not necessarily cheaper. Only budget hi-ten steel models are cheap. You'd want a quality steel frame if you have skinny wheels and value ride quality or if

It's not necessarily cheaper. Only budget hi-ten steel models are cheap. You'd want a quality steel frame if you have skinny wheels and value ride quality or if you're a tourer and want a bike that has many attachment points as is weldable in any shop or if you're a BMX-er and throw your bike around all day long or if you're a bike messenger or a fixie type and want a long lasting serviceable bike or if you simply like the looks of skinny tubes.