The New Zealand teenager's role on standby for both Red Bull Racing and its sister team Toro Rosso meant he was more statistically likely than any other reserve driver to step onto the formula one grid in the near future.
But the energy drink-owned outfit announced last week that Jaime Alguersuari, a Spanish teenager, is taking over for the rest of the 2009 season.
Hartley, contesting the Formula Renault 3.5 series as well as Euro F3, told the New Zealand press agency NZPA that his calendar had become too busy.
"That's the reason we decided to stop," he said.
"I learned so much doing F1 duties and it was such a great experience for me to see how everything works, but to continue would have meant zero weekends free until November.
"I really appreciated doing it, but I haven't really had a good result this year so I need to focus on my racing," added Hartley.
He wishes Alguersuari well for the role, but hinted that acting as reserve even for four of F1's twenty cockpits is not a silver bullet in terms of preparing for a career on the grid.
"Jaime will have that opportunity to learn now, but I am not sure what options there will be for him to get into the car," said Hartley.