To start off quickly with two things: This is not my main account which is why this account is only level 11. Second, my main account is ALSO low(ish) elo, whereabouts in low elo? That's for me to worry about. Point is, this thread and the information here is based on things I find myself doing as well as my teammates/opponents that I can see as objectively hindering our elo/gameplay. These will also avoid the obvious like "you're not CSing well enough" or "you're map awareness isn't good enough", those are quite universally widespread amongst threads that explain to lower elo why they're there. Lastly, these are all hypothetical, every player is different and these are just trends I see, so I can't say this absolutely does happen to you, but likely than not, you will see at least one of these present in most of your games in low elo.
To start off, here's the first reason why you're low elo:
When the enemy team buys certain items to counter you (e.g. Zhonya's Hourglass/Stopwatch), you don't take it into account until after they used it _to_ counter you.
Time and time again I find people blowing an ultimate or pulling off a risky dive only to be met with, in this case the best example is a Stopwatch under their turret. Whether the diver dies or not isn't really the issue, the issue is that people in low elo more likely than not don't register said item to be bought by their opponents until after they've used it.
Moral of the story: Your gameplay is probably chasing after the opponents' itemization rather than leading ahead of it to gain said upper hand.
Second reason why you're in low elo:
League of Legends in never a passive game. The only real way for games to be won in League of Legends passively is if one team's natural skill level is far above the other.
When I say passive, I don't mean passive gameplay like sitting back and sticking to CSing for the first 15 to 20 minutes of the game. All champions are different, some champions suit passive gameplay and some suit aggressive gameplay. What I mean by passive, is the notion of "going with the flow passively". More often than not, even when big decisions need to be made with coordination such as whether to and/or how to control the lanes for objective control or whether to do a 5 man siege or do a 1/3/1 push, games in low elo can go by without a single word being spoken in chat. No one to do the shotcalling, no one to properly declare big decisions. This more or less is due to two main reasons: fear of calling it wrong (and thus being blasted by your team) or laziness to take up the shotcalling role. Honestly, in low elo, 'some' shotcalling is better than none. Since low elo is already quite chaotic and unpredictable, more likely than not, the 'wrong' call can still win against 'no' call.
Moral of the story: If its getting to a point where big decisions need to be made such as Elder Dragon, Baron, sieging tactics or whatnot, don't be afraid to at least ask if you're still afraid of calling the shots. At least provoke or motivate dialogue.
Third reason why you're in low elo:
You probably don't actually care about winning the game. You more than likely care about winning an argument with an ally so you can have "justification in your head" why you lost a game.
Time and time again, I find games lost not due to an initial error in decision making such as a mistimed ultimate or an uncoordinated engage, but rather to the 5 to 10 minutes of people being AFK typing away why what went wrong. People much rather "lose a game with emotional justification" than "win despite feeling wronged in an argument".
Moral of the story: Actively notice what you're wanting to win. Do you truly want to win the game or are you comfortable just settling for that won argument?
Ultimately, improving your CS, your map awareness and your mechanics are all incredibly important, I feel like those points have either been done to death or are dead obvious anyway. Hopefully if you're low elo (like me) you can take something away from this. I know for sure that I'm no perfect player and I still fall victim to these pitfalls from time to time (specifically the item one) but shouldn't we all take a look at ourselves as objectively as possible to improve?