One of the biggest advantages that an LCS player has on the rest of us mere mortals is the coaching staff behind them. Frequently, these players have their champion picks dictated to them by a sixth member of the team whose sole responsibility is to be as knowledgeable about the game as possible. Thus, when said coach tells a player to put in a ton of games on, say, Ryze, he usually would explain to him exactly why he wants him to do so. It would be the responsibility of the coach to understand why he thought that pick would be good, and communicate that to the players.
Unfortunately, the vast majority of us in solo queue don't have any such figure to help dictate the metagame to us. For many of us less fortunate individuals, the words of the casters on the LCS stream is the closest we'll ever get to a professional coach, and thus our understanding of the metagame remains, at best, incomplete at all times. This becomes a pretty big issue when difficult to play LCS picks start winding their way into solo queue. It might be quite easy to understand the strengths of Syndra, and easy enough to execute on them, but the traits that make support Malzahar such an important pick in competitive play are often less clear, especially since it's a role that gets substantially less attention over the course of a game.
An excellent example of such a champion is Ryze, a champion that is widely considered to be either the best or amongst the best mid laners in the game right now for competitive play. For solo queue, however, Ryze is one of the worst possible champions you can pick, and tends to boast a mere 45% winrate at any particualr moment. Much of his weakness in solo queue stems simply from the fact that he's both a scaling pick and mechanically difficult to play, which depresses his win rate dramatically in a game mode where you cannot rely on getting to the late game with any measure of consistency. The more important factor, however, is that players simply don't understand what makes him so powerful in competitive play, and don't appreciate that those strengths aren't as pronounced in solo queue. In the case of Ryze, he brings two very powerful tools that are much less powerful in solo queue than competitive play.
The first of these is Realm Warp, his semi-global teleport ultimate. Global ultimates on a whole tend to be weaker in solo queue than they are in competitive play, simply because coordinating them is far more difficult. This is doubly true for an ultimate like Ryze's that is limited in range but infinite in application. Even amongst LCS teams there's a marked difference in how effectively Realm Warp gets used, with the better teams tending to find far more degenerate applications for the skill than the lesser teams ever can.
The second example is Rune Prison. As bizarre as it may sound, Rune Prison is a far more powerful spell for competitive players than it is for weaker ones for one simple reason: it's comparisons are all worse. Rune Prison is effectively the only point and click stun in the mid lane, with the only other one being on the far off-meta Annie. In solo queue play, where players are worse at juking and skillshot-based crowd control can be more heavily relied upon, Rune Prison actually loses quite a bit of value as it is, on the face of it, not a terribly powerful crowd control when compared to something like Charm, which effectively always kills any target that is unfortunate enough to get hit by it. In competitive play, however, these skillshots have a far lower chance of hitting as, in general, competitive players are extremely skilled at juking them. You can't juke Rune Prison.
All of this is to say, the next time you see a champion crest the win rate charts, spend some time and energy researching why before you blindly take them onto the Rift. Some, like Top Karthus, can lose the game for your team instantly if you pick them in the incorrect situations, and no one wants that.