Meh. Lucas thought it would be more thrilling to have Maul wait until the last second to show up and try to stop them from leaving. It's no more complicated than that. He wanted to give Qui-Gon and the Royal Starship a last-second escape for the sake of the audience. 
An IN-universe explanation, though, is always interesting(if frustrating) to think of. Kind of makes you wonder why Maul didn't go into Mos Espa. Nobody would have known he was a Sith Lord. Even with his dark cloak and facial tattoos he would have been no stranger than a lot of other creatures and beings in the spaceport and would easily have blended in, and the two Jedi were as of yet unaware of his very existence, much less his appearance. He could have tracked them down on foot and with his speeder...it's possible the Jedi might never have gotten as far as the Podrace had Maul been a little more thorough.
But who knows, maybe Sidious/Palpatine gave him explicit orders to remain in the shadows until he located them. That's always a possibility.
An IN-universe explanation, though, is always interesting(if frustrating) to think of. Kind of makes you wonder why Maul didn't go into Mos Espa. Nobody would have known he was a Sith Lord. Even with his dark cloak and facial tattoos he would have been no stranger than a lot of other creatures and beings in the spaceport and would easily have blended in, and the two Jedi were as of yet unaware of his very existence, much less his appearance. He could have tracked them down on foot and with his speeder...it's possible the Jedi might never have gotten as far as the Podrace had Maul been a little more thorough. But who knows, maybe Sidious/Palpatine gave him explicit orders to remain in the shadows until he located them. That's always a possibility.
