Mainly because Madara lacks serious feats that puts him onto Rikudou Naruto and Rikudou Sasuke's level. I think, by using hype and deductions, that he could be able to take on Sasuke without the Bijuu chakra or Naruto without excessive nature energy. Every stronger variant of these two take him low-mid diff, progressively decreasing with stronger versions.
But he can't take on them both, even with the restrictions above. Each of them are better versions of him (Naruto has better Senjutsu and cooperates with the Bijuus, Sasuke relies on his ocular powers and works on their weaknesses unlike Madara that spams Gudoudamas or CTs), while Madara has both eyes and body he isn't as good as them. Quality vs quantity basically.
