To Scar, Nuka was seen as a failure in his eyes. He never loved Zira at all. But he needed a son, so that's why he mated with her. But she produced a failure. Nuka... he wasn't right, 'perfect', as least in Scar's eyes.
That's why he loved Reno. Reno was perfect to him. He was everything Nuka was not. Scar had big expectations for his second son...
... then Simba came along...
Lineart (C) LineartsBasesTemplates
Scar (C) Disney
Reno (C) Me