The easiest answer is "Because balancing the CV Rework in Random Battles was a rough experience that we don't want to repeat."
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When Reworked CVs entered the game, they went directly into Random Battles, which was a rather jarring upheavel for players. For one, there was still a lot of balancing/iteration to be done on the class, and two, because it was a sudden change from one day to the next for those that don't use the PTS or keep current with the news.
After learning from that, Submarines have been tested in a very long-form fashion. Multiple behind the scenes tests, then PTS testing, then a Submarine Battle mode on Live Server, then more behind the scenes testing, then more PTS testing. What's been found is that the PTS simply isn't treated as a real game server, so matches don't play out very well for playtesting/understanding purposes. Similarly, the Submarine Battle on live server was treated as a side-mode and gave some information... but matches weren't played seriously enough to get a sense for what Random Battles will see. So, another non-Random Battles option was needed to be able to see if the ships were ready to be tried in Random Battles.
Ranked is a mode which is NOT Random Battles, and is Voluntary. If you don't want to play Ranked, or want to skip a season, you can choose not to take part. However, for those that DO play Ranked, they really do want to win. This gives a place where people will play as best as they can, and the new things can be tried out in a "real" environment, at least enough to make sure the mechanics and interactions are working and can give good information to our Design team.
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There's been a lot of feedback given about the frustration of Ranked being used as a Testbed. We've heard you loud and clear on that front, but at the moment it's the best option to use while keeping Randoms isolated from newer design concepts while they're being ironed out and to make sure everything is working correctly.
It is unfortunate that there's been so much to test in such a short period of time, and that Ranked Battles have borne the weight of being that Testbed for the recent months. Please know that we are hearing your feedback about wanting Ranked without any Test conditions and that we understand the frustration felt.