While it’s hard to chalk up all of that difference to the fact Keanu Reeves wasn’t in the movie, Reeves was certainly a bigger name than Jason Patric, the guy who took the role of male lead opposite Sandra Bullock in the sequel.
Also, the simple fact that Reeves didn’t return could have potentially signaled to many moviegoers that the follow-up wasn’t worth their time.
If Reeves was the reason you saw the first one, why bother with the sequel.
A brief look at the box office results makes it clear why Fox was unhappy.