What is the rule behind 一带一路?
It is a title of a project, not a sentence. 

「一带一路」」refers to "The Silk Road Economic Belt and the 21st-century Maritime Silk Road"

「一带」(the one belt) refers to 「丝绸之路经济带」

「一路」(the one road) refers to 「21世纪海上丝绸之路」

Both are specific object, therefore the title cannot be just 「带和路」(belts and roads)

Using 「一个地带和一条通路」would be too wordy for a title. Not to mention 「丝绸之路经济带和21世纪海上丝绸之路」