Two reasons why it failed in the U.S.:
1. It came too late, the Catera was released in the US in 1997, while the Omega was released in late 1993, which made it a bit "old".
2. It had no V8. The American's are V8 freaks, and as long as a Car is RWD, they obviously wan't an V8 Version, which nearly happened in 2001, but then they realized, that the European's don't wan't an V8, and only weeks before it was about to launch, they killed it off, and with it, the Catera. The Omega then, was finally killed off in 2003, for reasons I quite not understand.
But if we look at it now, then the simple answer to it would have been, if Cadillac rebadged from the begin on the Hold Commodore VT as Catera. It has a remomodded Omega GM Y platform, was released in 1997 and had acces to an Corvette V8.