Amitabh Bachchan did not dominate the 70s and 80s , its a myth created by the media.
Well, as far as I understand, Amitabh completely ruled the 70s and 80s Bollywood.
Yes, he had a lot competition to deal with. The likes of Rajesh, Dharmendra, Vinod Khanna, Rishi Kapoor all had their moments of greatness.
However, the sheer versatility and range that Amitabh possesed during that era remains unmatched, and that is one of the main reasons why he is still so admired when the rest of his peers had completely vanished from the scene a long time ago.
Zanjeer, Sholay, Deewar, Amar Akbar Anthony, Don, Muqaddar ka Sikandar, Kabhi Kabhi, Naseeb, Kaalia, Coolie, Mard, Shahenshah... these are some of his classics which released in that era and completely separated him from other Bollywood stars.
I recall one French director even calling Amitabh Bachchan a "one man industry" during those times.
The main difference between him and other was the fact that he was not limited to a single genre.
Rajesh Khanna and Rishi Kapoor were noted for their romantic images. Vinod Khanna and Dharmendra were the action heroes of that era.
Amitabh Bachchan was a master of all trades. He could portray the angry young man image in a film, while adding his comic timing and romantic lines into it and making it a complete "masala" film, something the names mentionned above were not capable of doing.