Why Does Ethereum does not use Delegated Byzantine Fault Tolerance? has Vitalik considered it before?
(Possibly an answer based on conjecture... )
The clue is in the name. Anything "delegated" involves a degree of centralisation that likely exceeds what Vitalik originally had in mind.
The same line of reasoning has likely led to the development of Casper rather than the use of existing Delegated Proof-of-Stake consensus algorithms, which provide higher throughput at the expense of security.