Between those two movies the differences were originality and timing.
In Aliens we had the same villains as in the original, but James Cameron gave it an action-movie treatment (which was new) and the story tossed in some twists of its own ... a android being a good guy, the aliens' giving the colony their version of an extreme makeover, and the alien queen (the biggest twist of all!).
Aliens also benefited by fortunate timing, in that there was a seven-year lapse between the original (1979) and the sequel (1986), which is like a generation in moviemaking time. People who saw the original will encourage their younger siblings and friends to see the sequel; people who were too young to see the original in theatres will go and check out the sequel. (The latter group includes me, as I was 12 when the original came out.)
Conversely, Event Horizon came off like "The Shining" in space. The way it was written, to really get into the movie you'd have to be both a sci-fi AND a horror movie fan. There's probably a fair amount of overlap between the two, but when the movie axiomatically narrows down the target audience, that doesn't bode well for how it'll do at filling movie theatre seats.
Happily, time has been kinder to Event Horizon than the box office was, and it's now recognized as an underrated classic by hard-core fans of both the sci-fi and horror genres. EH also benefited from the DVD era ... the 2006 Special Edition DVD has some deleted footage and a LOT of "making of" material which help to explain the story and make it easier to appreciate.
Also conversely from Aliens, Event Horizon suffered from decidedly UNfortunate timing. Remember I said it came off like "The Shining" in space? Well, a TV miniseries remake of The Shining aired in April-May 1997, and it was pretty good. Then along came EH in **August** 1997. I can just picture the first crop of viewers going to EH in theatres, not being terribly impressed, and telling their friends, "It's kind of lame and it just rips off The Shining, I wouldn't bother plunking down six bucks for it if I were you." That won't help box-office receipts either, obviously!
There's nothing anyone could have done about the timing issues Event Horizon faced. As for originality, I suppose they could have tweaked the story so that the Shining connection wasn't so blatant. Problem is, though, The Shining (1980 movie) is such a classic that escaping the connection completely would have been well-nigh impossible!!