Why is Bitcoin Core using LevelDB instead of Redis or SQLite?
Redis and LevelDB solve very different problems. We tried using SQLite and its performance was abysmal.

Bitcoin Core needs a database to store the set of unspent transaction outputs (UTXOs). This means we need fast simple reads, and fast batches of random updates.

We don't need a server/client architecture, as we can't have multiple applications accessing the database at once anyway: inconsistencies in the database would lead to forking risks (every node in the network needs to make exactly the same judgement about what is valid and invalid).