I believe there is another thread telling the story of St Helena airport on here. But to summarise the airport was designed to cope with a 738 as the base case. It's in the middle of the south Atlantic and will only get one or two flights a week near term. Funded by uk tax payers to reduce dependence on aid it connects one of the last outposts of the world to the global airline network.
This was not the first flight as a King Air navaid calibrator did that but it is the first large aircraft. The single weekly scheduledflight is to Johannesburg and will see the retirement of the mixed pax cargo ship which has provided the only access for decades.
Can tourism work on a tiny island with few attractions except isolation and exclusivity who knows but the airport construction, an expensive task anyway was greatly complicated by lack of any flat terrain requiring extensive earthworks.
Diversion is back to SA or Namibia or closest of all the US airbase on Ascension Island St Helenas nearest neighbouring an hour's flight away but with a nice long runway.