Leimer finished the penultimate race of the season fourth, which was enough to give him the title by a point before Sunday's finale as Bird could only recover to finish 10th.
Bird blamed an overheating clutch for his startline issue, and also said it hampered his pitstops thereafter.
"We were lucky Sam stalled on the grid, which helped us a lot," said Leimer. "I feel really sorry for him - he's a really nice guy. I think he also deserved to win the championship, but in the end it can only go to one driver, and luckily it was me.
"The start was good and I was almost P1 [at the first corner]. We knew we had to finish at the front to take the title, but [Bird's stall] took off the pressure.
"I still can't believe it, it's just amazing. I am really happy to win this championship with Racing Engineering because these guys work so hard."
Following his success, Leimer is unsure of his next step, but hopes that he can find a place in the Formula 1 paddock.
"I hope I can go to Formula 1," he said. "It will be hard, for sure, because there are not many seats free, but I hope I can go to F1, and for me it is important to win this championship to help me.
"Now we have to work and push to go up there. Of course it would be fantastic to be a race driver or even a third driver, but we will have to work hard and then we'll see."
For his part, Bird shrugged off the disappointment of losing his title shot.
"I think second in the championship is still a fantastic result with a new team," said Bird. "Unfortunately, the clutch did what it did. But we've won five races this year, and that's no mean feat."