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

The concept of making a logical representation of data abstracted from its physical representation was perhaps the most game-changing aspect of Codd's idea. He was apparently the first person who fully realised the benefits of separating logical and physical concerns and therefore the first to devise a data model worthy of the name. By describing a model based on relations, without navigational links or pointer structures he also created something uniquely powerful, flexible and of lasting relevance.

To be accurate it must be said that it was the SQL model rather than the relational one which eventually proved more successful commercially. SQL is a long way from a truly relational data model or language even though it would not have come into being without Codd's ideas to inspire it. The relational model's creator was naturally disappointed that SQL rather than relational became the database standard. Four decades later I think we have plenty of cause to regret that Codd's relational model isn't better supported by DBMS software.