The main factor was the fact that the 747s were getting very long in tooth and expensive to maintain; they were 1970-71 vintage.
They looked at various options at the time, including secondhand 747s (I believe secondhand SAA 743s were considered), new 767s and of course, A330s. I believe Colm McCarthy, who later joined FR, was in charge of the evaluation.
To cut a long story short, the A330 cut all of the other options to shreds; it's economics were FAR better than those of the 747 and its cargo capacity was another big winner.
The A330 choice was definitely the right one. It was and remains one of the most economical, if not the most economical t/a airliners. The early -300s were a bit short on range, although they could still make ORD, which is an 8h+ trip in Winter, but the newer -302E models do that fine. I think it's fair to sat that the 330 itself was not the cause of any of EI's problems; for example, on the DXB route, the 330 COULD have been a big success, but EI didn't have the marketing, scheduling or consistency of product.
I strongly believe that in the current market environment, long haul low cost is the way to go and the cost advantages of the 330 might make that a very attractive proposition. The SNN-JFK route had some of the lowest t/a LFs for some years, so it's not a big surprise that it's being cut. I always found it difficult to understand how the route supported CO (to EWR), DL and EI.
I do have concerns about the 359, which I think will have to be revisited - and probably is being, as part of the "future size and shape" project, but I strongly believe that the 330 is one of the finest aircraft EI has ever introduced.