According to the Kotor port schedule, the Sun was to be in Kotor on the 11th, again on May 6th, June 17, July 29, August 19, September 9th. The Sun Princess at 175,000 tons is by far the largest ship on the port schedule. http://www.portofkotor.co.me/
I suspect, as previous posters have pointed out, is the ship is too large to navigate the channel. Celebrity Edge is slightly shorter and slightly narrower than the Sun, and is 140,000 tons and only calls on Bar, not Kotor for every Montenegro stop this season.