It is for the following reasons:
1.SasuSaku is canon
2. Some people started shipping ObiSaku when Sakura and Obito were talking inside of Kamui's dimension and when they were talking while they were trying to save Sasuke. It didn't help that Obito was half naked and Sakura kept losing clothes in that fight.
3.I first saw ItaSaku in thee days of road to ninja when alternate world Itachi grabbed Sakura's arm. Also, Itachi is the 3rd person after Naruto and Madara that people like to compare Sasuke's accomplishments to, so people probably said "What if Sasuke in SasuSaku was replaced with Itachi?".
4. I don't know specifically how MadaSaku came about, but I first saw it come about in the days when Obito was still posing as Madara and wearing that orange mask.