From Wiki;
Rational choice of scales
The nominal height of a man is simple in the inch-based system: six feet. Many traditional scales are derived so that a figure of such a height against the model can be readily imagined as a simple relation to an inch. Although the metric system has specified a limited series of scales for blueprints and maps, when it comes to models, there may be a problem with these scales for a readily imagined person of 180 centimetres. Model railways have the additional difficulty of having to present the rail gauge as a simple number, the height of a person being secondary. Trade authorities in metric countries are attempting to specify scales that are simple mulitiples of 2 and 5, but neither tracks nor people seem to fit. In such cases, rationalization may actually be invoked for competitive advantage, to prevent interoperability with products from another manufacturing country.
On the other hand, wargaming scales have traditionally been traced to metric system, where the number of millimetres relate to the relative height of the human figure based on 180 cm standard man. Therefore 25 mm scale (popular in historical and fantasy wargaming) refers to 1:72 scale, whilst the 15 mm scale (nowadays the most popular scale in ancient, medieval and Renaissance wargaming) refers to 1:120 scale (Many manufacturers refer to 15 mm as 1:100 scale). Likewise, 50 mm scale is the same as 1:35 military model scale, and 5 mm equals 1:350 naval scale.
Rationalisation typically falls into 2 categories: Industrial Trade, and Hobbies. Industrial trade covers areas like building and maritime architecture where universally accepted scales have been established and are conformed to both in the construction of models and drawings, in order to facilitate smoother co-operation between commercial parties. The rationalisation of hobby scales has been more gradual and organic; largely driven by clubs lobbying industry, as well as tradition, and indirectly, consumer demand. A couple of artificial efforts to standardise have not been successful: ie Tamiya in the 1970's with aircraft models in 1/100th scale, and Heller with airline models in 1/125th scale. Even though rationalisation in hobby scales sounds appealing, it has not proved to be historically evident, although there may be some very gradual progress as consumer demand becomes more organised.