Terranigma not being released in the US is exactly why it's an unknown title outside of its cult fanbase. While it was released in PAL regions at least so no need for fan translations. As for the reasons why it was never released? It can be chalked up to two reasons or perhaps both:
The game has more than a few religious references scattered through the game that probably wouldn't have gone over too well in the US back then.
The much more likely reason though was that during development Enix had closed down their US branch, meaning that for a few years no Enix published games reached North American shores.
It's a shame too, Terranigma is such a outstanding game to the point I liken it to the Chrono Trigger for PAL regions. (at least until the DS version came out) Another thing about that is us Europeans (and fellow Aussies too) never received any of the big SNES RPG's like Final Fantasy VI, the previously mentioned CT and FFIV. Hell, PAL regions didn't even get a Final Fantasy title outside of Mystic Quest until Final Fantasy VII!
As for the rest of the Soul Blazer/Heaven and Earth Trilogy of which TN is a part of, Soul Blazer was the first title in the series, and until years later I honestly didn't know it existed. From what I've played so far, especially compared to the latter two titles, it's just... Not very good. Definitely hasn't aged very well, in my own opinion of course.
As for the second title, Illusion of Time (Illusion of Gaia everywhere that's not Europe and Australia) I owned this first before TN came out, and it's a damn good game in it's own right. It's the most linear of the three but it has the richest characters and world of all. I personally think it's just a few hairs behind Terranigma in quality. The only thing that marrs it in my opinion is the bad translation.
As for Terranigma, the music is crazy good, settling it alongside some of the best SNES/SFC soundtracks. But enough gushing, the game does have it's own (minor) problems, such as the weird leveling system. You can reach some areas of the game at a decent level but the enemies can and will wreck you unless you raise up a few levels, then suddenly almost everything in the area is your *****. One dungeon and boss is an infamous example of this. It just can feel a little unbalanced is all. Another is that in both IoT and TN, if you want to 100% the game you pretty much just might need a guide for your first play-through as there are a ton of missables. This wasn't uncommon for the era though.
In the end, if any of you reading this have a passing interest in Terranigma (and the other two titles in the trilogy) then keep in mind your only options are to buy the cartridges at an exorbitant price, and in the case of Terranigma you should keep in mind that if you are buying the European (black box) version there's a pretty good chance you won't get the English release. A much safer bet is getting the Australian (white box) version instead. And also you will of course need a PAL SNES or alternately modify your existing machine to take PAL cartridges. There's also reproduction carts modified for playing on a US SNES that you could buy for a much cheaper price, but I hear these versions have problems of their own such as a lousy internal battery. Other than that the only other option available to you is playing them on an emulator as they have not been made available on the Wii, 3DS or Wii U Virtual Consoles. ActRaiser, Quintet's debut title is available on the Wii VC though and is an interesting game in it's own right. It isn't part of the Soul Blazer trilogy however.
Information on Terranigma and the other three titles. Spoilers abound in the plot section of each. Avoid!:
http://en.wikipedia.org/wiki/Soul_Blazer
http://en.wikipedia.org/wiki/Illusion_of_Gaia
http://en.wikipedia.org/wiki/Terranigma
Information on the developer, Quintet:
http://en.wikipedia.org/wiki/Quintet_%28company%29
One final comment: There is one more game that is considered to be the final entry in the "trilogy", called The Granstream Saga for the PS1. While I've never played it myself it is apparently pretty bad to average at best, and was made by Shade, a devteam consisting of former staff from Quintet. because of this fans don't consider it a part of the Soul Blazer/Heaven and Earth trilogy.