Because people in S rank are better than you. It's that simple.
You're not "bad" at this game. Being in A rank or higher, in my opinion, is when you officially can say you're "good" at Splatoon. The skill limit in this game is very, VERY high. S has a wide spectrum of ability. Some players are just clawing their way out of A+. Others just had a bad run and dropped from S+. Even if you're getting destroyed in S, you can LEARN from those defeats. Just take a moment and think. "What am I doing wrong?" Odds are your positioning needs work and you're not approaching properly, setting yourself up for failure. Or maybe you're wasting your specials. Not watching the map enough. Or your movement isn't as crisp as the other players. Something needs improving, and you can identify what it is and work on it.
When I first got into S rank, I had only de-ranked once. From A+ to A, and I won every game in A on my way back to A+. But since achieving S, I've probably de-ranked four or five times. It just happens. But I have improved A LOT. I used to, in all honesty, get carried quite a bit in S. But then I started holding my own. Nothing spectacular, but going pretty much even in kills. Every time I've reclaimed my rank, I've done even better. Since re-claiming my S rank on the most recent occasion, I have actually been dominating. I've lost quite a bit, but putting more time into the game and just grinding out those losses in S rank has DEFINITELY improved my play. When I de-rank, it's like loosing a lion in the gazelle exhibit. Except I'm the lion and the poor A+ players are gazelle. Maybe I'll de-rank again. But I am so much better now than I was when I first achieved S rank.
You're in the process of that right now. Don't get discouraged. Almost everyone goes through a bit of a struggle when getting into S rank. If you put in the time and the effort, with a good mindset you will improve. The most important thing is to have fun. I know, I know, shocking, but you have to ask: Why play Splatoon if you don't enjoy it? Find what makes Ranked fun for you and do that. Personally, I love ranked. Winning, losing, whatever. A concrete objective to push instead of just inking the ground? Sign me up! Something that keeps it fun for me is developing different builds and playing different weapons. I have a pretty large stable of weapons I like to use for different mode/map combinations and different playstyles. Maybe using more weapons would help you! As long as you're enjoying yourself, you're playing the game properly. Just find out how to do that.