There's a theory[0][1] that it's mostly down to aesthetics. It looks visually more pleasing that way when split into three groups of four numbers:

   I, II, III, IIII (consisting of I only)
   V, VI, VII, VIII (consisting of I and V)
  IX,  X,  XI,  XII (consisting of I and X)

[0] http://mathtourist.blogspot.co.uk/2010/08/iiii-versus-iv-on-...

[1] https://www.hautehorlogerie.org/en/encyclopaedia/glossary-of...