When Smallville was still on the air, there was talk of Oliver Queen (Justin Hartley) getting his own TV series, so why did the CW create Arrow with Stephen Amell in the lead role instead? Long before Amell wore the green hood, the CW had Justin Hartley as its Green Arrow on Smallville. Introduced in Smallville season 6 as a recurring character who could push Clark Kent (Tom Welling) closer to fulfilling his destiny, Hartley's Oliver Queen became a series regular for the show's last three seasons.
At the end of Smallville's 10-season run, the show's success led to the CW launching another superhero series based on an iconic DC Comics character. When it was reported that the network was looking at Green Arrow for the show's protagonist, naturally some fans expected that a spinoff was coming for Justin Hartley's take on the Emerald Archer, but this never happened. Once Smallville ended, Arrow went into development, and new actors were cast, with Amell playing Oliver Queen. It was later revealed that Hartley was offered a cameo on Arrow on multiple occasions, but it never worked out.
If the CW already a Green Arrow with Justin Hartley, why did it recast him with a different actor? It's true that the CW did in fact want to launch a spinoff about Hartley's character, but by the time Smallville ended, a new approach was chosen. According to Hartley, numerous options were explored. After Green Arrow formed Smallville's Justice League in season 6, the CW considered creating a Justice League series centered on Oliver as its leader. Another spinoff idea was a show that would have sent Oliver and Lois Lane (Erica Durance) - with whom he was romantically involved in season 6 - to Metropolis. But the creators of Arrow wanted to start a new story from scratch, rather than continue an existing one.
Arrow executive producer Marc Guggenheim said in 2012, "We certainly wanted to chart out own destiny" [via EW]. Arrow season 1 told a story about Oliver Queen coming back from being on an island for five years and going on a mission of revenge to kill all on the people on his father's list. From the beginning, Oliver Queen was a very different kind of hero than what fans were used to from shows like Smallville, as Oliver was a dark character who was willing to kill for his mission, but over the course of the series, Oliver's mission evolved and his methods changed. Arrow couldn't have gone in this direction with Smallville's Oliver.
Smallville already told Green Arrow's origin story, explained how his parents died, married him to Chloe Sullivan (Allison Mack), and had him deal with his own issues about the morality of killing, whereas Arrow's Oliver Queen had yet to go on any of these journeys. With Stephen Amell's character, Arrow gave the writers and producers an opportunity to create new characters and new conflicts. By starting from scratch, Arrow was able to provide eight years of stories for the Emerald Archer.