Here is a recent blog post by IOHK explaining the different "flavors" of Ouroboros.
Regarding your question/comments - there wasn't anything "wrong" with the Ouroboros-BFT (OBFT) concept of federated servers. As per the original Cardano roadmap, the network was intended to be bootstrapped from a federated setting (where all block production is done by a central entity, in this case IOG), and then transition to a decentralized network (Shelley Era). This transition had to be executed well, because every transition/HFC after this point would be on a decentralized network, with no chance of reverting back to a central point of control.
Without going too deep into the differences between O-Classic and OBFT (because I don't really know them), OBFT was a sort of cleanup of all the lessons learned from Classic that would allow for smooth and stable transition(s) from Byron to Shelley to Goguen and beyond. In other words, it was extremely important that the initial conditions of the system were such that, upon releasing it into the wild, evolving/upgrading the protocol would be as straightforward as it is today. A lot of forethought went into this, and for this reason, the classic to OBFT transition was the only real "Hard Fork" in Cardano's history. It marked the end of Cardano's infancy, and imbued the system with the ability to evolve.