I doubt that the Post office railway precluded a station at Mount Pleasant - it wasn't a problem at Liverpool Street or Paddington, and anyway the Circle Line was there first. I had assumed the reason was because the Metropolitan railway's original raison d'etre was getting people from the main line stations to the City. Clerkenwell is walking distance from the original terminus at Farringdon so there would have been few takers for the service, and the railway didn't want its services clogged up with short-hop passengers anyway. There are other examples of long gaps between a terminus and the first station out - see Deptford, New Cross, Finsbury Park, Acton Main Line for examples.