mysql is slow in terms of read performance
Why not use leveldb as used by core. It has very good read speeds. Also you don't require concurrent writes. Only need to write a block that comes in every 10 minutes. Most of the times are reads. I doubt any other RDBMS can match the performance of leveldb
Because LevelDB is not ACID compliant and prone (read, renowned for) corrupting the DB. This is why we are being told to back up the chain, make copies etc. Bitcoin should be using an ACID compliant DB and the only one that gets close to the performance of a Key/Value DB for sequential access is SQLite. Plus, one has other niceties like live backup and in memory attached DBs.