I'd suggest that smaller aircraft offering more frequent services to be preferable to larger aircraft offering less fequent services particularly taking in to consideration the terrain and weather conditions associated with St. Helena.
So what if the runway may be capable of accommodating B737-800's & A320's, why take unecessary risks if B737-600/700's, A318/319's, Embraers & Bombardiers and whatever can do it just as comfortably and with less risk involved.
But there is a modern day mentality to squeeze as many bums in to one flight as is possible, a local airline to me (Air Philippines) have realised the error of their ways, their Q400's don't have the runway performance to operate to/from a number of airports here so they've needed to go back in time and recommence Q300 operations.