I think St Helena is still building its airport with a short runway so maybe Ascension is a fuel stop.
No, that's not it.
The runway is indeed still being finished, however the length is fine.
The entire Atlantic Star business model, such that it is, is built around direct services.
The 'officially endorsed' / contracted service will be Comair to JNB.
Two issues with this, for leisure, it means a connection.
For Saints, they would prefer to be connected to Cape town, where they have established family links rather than Joberg.
As said, the atlantic star business model is based around direct UK(london) connection to st helena.
But even though leisure travel is hoped to be a big new market for the island, this is not mass market, and it is not normal market, this is a very niche type of business we are talking, and heavily skewed towards top end price wise.
What Ascension is, is it is the closest divert. However, currently you cant designate it as a civilian aircraft (only VERY limited nonscheduled civilian traffic is allowed). Hence the Commair service will use airports in Africa as the divert.
Atlantic Star hope to negotiate better access to Ascension island. And for there they could onwards serve the Falkland Islands.
But Ascension is caught up in a huge range of different issues.
Any venture such as this is likley to be highly speculative (without a contract to provide service for someone). So we really can not apply any standard models to examine them.