Volare
As i understand it the problem is freight. St helena for years has had a regualr cargo passenger ship dedicated to serving the island-everythign thta was imported had to come that way-and as asmall island everything has to be imported.
St helena is too small and too remote for regualr marine traffic 'passing by' and has very basic port faciltiies hence the special purpose vessel.
However it was decided that if an airline rana meaning scheduled passenger service with decent freight capacity thena few ship visits a year with heavy/non urgent non perishable stuff would be fine.
The problem with C series, Embraers as I understand it is they cannot carry much cargo compared to a 73-8 and the Boeing can also trade off pax numbers for more freight weight when pax loads are light.
Thats why it looks like they have painted themselves intoa corner if the 738 or similar cannot operate-the smaller aircraft are just too small.
Apologies if I have done down the capabilites of these smaller alternatives