The Scillies are a rather odd case because the ferry and the airline are operated by the same company. In general the ferry caters for more local traffic from Cornwall whereas the Twotters are aimed at the longer haul from Bristol, Exeter and London via Newquay. In season, however, it is almost impossible to get an air ticket unless you book months in advance so the ferry takes up the slack and passengers face a long road trip of, for example, 4 hours from Bristol to the ferry.
The Twotters make a good profit because they are full for most of the year.
This situation effectively limits the number of visitors to suit the limited accommodation on the islands.