Trivia: The director on the set of Johnny Cage's movie bears a pretty striking resemblance to cinematic legend Steven Spielberg. Spielberg, who famously is a big fan of video-games, was reportedly approached to film a cameo, but was unable to free up the time. Thus a lookalike was cast as a bit of an inside joke.
Trivia: Brief mentions are made of Liu Kang being a descendant of a warrior named Kung Lao, and a statue of Kung Lao can be seen faintly in the courtyard that is filled with statues. In the games, Kung Lao is actually the name of two characters - one an ancient and famous warrior, and the second one of his descendants, whom is both related to and good friends with Liu.
Trivia: The original script reportedly had much more violence, which the studio had removed for a PG-13 rating. The original script also made Liu Kang and Princess Kitana's potential romance a bigger focus, whereas in the finished film, it's more subtle and implied, and the romance doesn't fully play out. (Although they are implied to be in love and entering into a relationship by the end of the sequel).
Answer: Since the entire fight is focused on Liu Kang and Reptile, he could have possibly waited to see what the outcome of the fight was.