historically, what made relational databases popular?

Question

EDIT I\'ve just started skimming Codd\'s famous 1970 paper that started it all, that Oracle was based on (A Relational Model of Data for Large Shared Data Banks [pdf]),

Answer 1

From my knowledge, it is the normalization theory (the well known Codd's Third Normal Form) to define relational data model that is easy and efficient for storing and retrieving. This followed by the Standard Query Language (SQL) which allows it to be used across all the relational db system. Standardization was definitely lacking back then which also make this appealing to many.