Bing/atakacs...
Just to second Flap's comments whilst the Delta diversion is of interest I thought the question posed was the use of Ascension as a alternate on Europe - South America services.
Most of those flights tend to have a route which goes "feet wet" off Morocco or even further north, Routes over or abeam Tenerife, and then uses the likes of Dakar at the northern end and Recife or Fortaleza at the southern side as nominated ETOPS alternates... ETOPS planning aside I've never seen ASI as being anything like "nearest suitable" on those routes it is simply too far off to the east.
The Delta flight that did go there was obviously routing from a North America to South Africa, so not surprised ASI was more likely going to come into play.