I think that was because the developers put their weight on the multiplayer experience so the single-player part (story and skirmishes against A.I.s) ended up rather mediocre.
While this sounds like your everyday shooter released during the last 5 years, back in 2002 the reviewers still cared a lot about the offline experience.
That and the game had Unreal Engine 1 looks but it was released during the same year as Unreal Tournament 2003 (Unreal Engine 2), which was a bit off-putting.
The best part was of course the multiplayer experience with vehicles, something that would only appear again in Unreal Tournament 2004, 3 years later.