There are various reasons of course, but you first need to establish what makes a top level player a top level player.
Strong fundementals
sense of game knowledge
strong mentality
consistency
More or less these are the 4 main points, and other aspects of the game fall into one of those said 4 categories. Now some of these are easier to master or achieve than others, but the kicker is being good in all 4. The game is, more or less, split up in 3 different aspects of gameplay:
Neutral
advantage
disadvantage
Multiple different apsects of the game fall into one of the 3 whether it is your ledge trap game (advanatage), your mixups to reset neutral (disadvantage) your spacing (neutral, but important for all 3 phases), mindgames (all 3), or even minor things like punish windows and mu knowledge.
Ok ok. but you want to know not only what separates mid level players from the good ones, but how to really get there. Well i can't answer what each player needs to specifically work on, but i can lay it out to show how to reach the next level. Keep in mind this is more theorycrafting on my part, so keep that in mind.
Neutral- what makes you an amazing and good player. having a good neutral means you can bait, condition, read, space, react and whiff punish in neutral, mix-up and mindgame your opponent, and so on. NEUTRAL IS WHAT MAKES YOU A GOOD PLAYER
advantage- This is your punish game, and how much you can get off of your neutral wins. this involves your ledgetrapping and edgeguarding game, your punish and combo game, and so forth. it is in this phase that you take a majority of your opponents stocks. having a strong advantage state means your opponent doesn't have a chance to comeback or retaliate, and gives them less chances to cheese you or win. ADVANTAGE IS WHAT MAKES YOU A CONSISTENT PLAYER
disadvantage- This is your how good you are at resetting neutral, recovering, midigating damage, effectively landing and getting back to stage, survivability, and reversing or stopping your opponents momentum. It's players with good disadvantage and survivability that pull off upsets, have that magical "clutch" or "comeback" factor, and cheese stocks, games and sets away that they otherwise shouldn't have won. DISADVANTAGE IS WHAT GIVES PLAYERS "PLOT ARMOR" (for lack of a better term)
Keep in mind that some characters also naturally have tools that fit into one of these 3 apsects of the game better than others. Mario for example, has an amazing advantage state, while characters like duck hunt, may not. diddy and sheik have top tier neutral states, while robin, lucario, and ganon do not. you need to look at your characters strengths and weaknesses, and then look at your own. i'll also give some staple questions to help identify where you might be lacking.
Do you do well against players better than you or equal to you?
Are your placings consistent, both at locals or larger tournaments?
Do you pull upsets at all, and if you do, do you do it often?
Do you find yourself running into a "bad MU" often?
The first question will help identify if your neutral game is lacking. if you find yourself outclassed by better players, then your neutral game needs work.
The second question will help see if you might need to work on your advantage game, as that is what nets stocks.
The 3 question asks how good your disadvantage is, as better players can lose to an opponent who can survive long enough to cheese a stock or game (either through rage, or something else.)
the 4 question is pretty simple. it's possible your character is lacking, and you either need another character, or a new main altogether.
Again, this is a lot of theorycrafting on my part, but hopefully it's some good food for thought.