Random Battles is our primary mode and what we balance for in general. Submarines being tested in Random is important because we balance to Randoms as a standard.
Petro was banned from Clan Battles/Competitive for consistent over-performance in terms of Clan Battle data. She's been locked out for a while, but she has had two significant nerfs (she was raised up in the water and her armor was thinned) in the meantime. While we could have assumed she was good to go and just put her back in, we elected to see how the nerfs effected the ship generally and are observing her in her natural habitat of Random Battles.