The spinoff - provisionally titled Star Wars: Underworld - will focus on "the criminals and the gangs" of the Star Wars universe.
"We're at a complicated impasse right now," McCallum told IGN. "We have... 50 scripts [that are] unbelievable. The most provocative, the most bold and daring material that we've ever done."
McCallum added that Underworld will only enter production once George Lucas can make the show "at the cost of maybe $4 or $5 million an episode".
"The trouble is, they're so complicated," he explained. "Each hour has more visual effects and digital animation than any of the films that we've done and right now we're just struggling.
"That's our biggest challenge, that's our biggest problem. [The episodes] are so big, they're so complicated, and we just don't have the technology to do that."
However, McCallum predicted that Star Wars fans would eventually get to see the TV series in "a couple of years".
"[George] will find a way and once that happens I'm sure he'll start back on [the show]," he insisted.
> Star Wars TV series 'is Empire Strikes Back on steroids'
Watch a clip from The Empire Strikes Back below: