Great question! Unfortunately, there isn’t a very good answer...
English compound nouns
have three possible forms; open
(bike shop), hyphenated
, and closed
(bookshop), unfortunately, there isn’t really a rule describing which to use, although they tend to start off as open and eventually become closed.
e.g. according to http://www.guardian.co.uk/styleguide/h
“Inventions, ideas and new concepts often begin life as two words, then become hyphenated, before finally becoming accepted as one word.”
While Wiki, http://en.wikipedia.org/wiki/English_compound
“The "solid" or "closed" forms in which two usually moderately short words appear together as one. Solid compounds most likely consist of short (monosyllabic) units that often have been established in the language for a long time.” e.g. bathroom
“The open or spaced form consisting of newer combinations of usually longer words,” e.g. sitting room
But also points out that; “open, hyphenated, and closed forms may be encountered for the same compound noun”
Hope that helps,