What factors contribute to Leona not being chosen by pro teams in the LCS 2015?

The answer is simple - because others are better.

Thresh can engage from the same range, has a better laning phase and can peel a lot better than leona can. Braum has a reliable amount of CC and can peel quite decent too. Tahm is too powerful to let him through - Massive dmg without building anything and just awesome when it comes to protecting people. Jinx Tahm is the botlane at worlds which won the most games. Morg also has a good engage from a long range without putting herself into danger, and the black shield is a blessing for every adc. And Janna is the queen of disengage, basically you can't build a pokecomp without her.

Almost every champ in league is usable and good if you know how to play him. If he's not in the meta, it's simply because others are better.

The short answer is: Leona does not fit into the meta at this time.

Keep in mind, when we talk about meta in this context, we're not talking about what's effective in solo queue, we're talking about what's effective at incredibly high levels of coordinated team play. Team composition is the only thing that matters at high level play. The champions are all in a fairly balanced state at the moment. The way they are distinguished from other champs in pro play is how well they fit into a certain composition.

Why the Other Champions?

Thresh - Thresh is great for making picks and saves. His hook and his lantern provide a ton of utility to all phases of the game. A good Thresh can set up fantastic engages and greatly hinder diving on their back line with the box. His Flay also cancels dashes and many other gap-closers and escapes.

Braum - Braum's passive is just insanity. On auto-attack, he sets up a medium stun after 3 more autos from any of his teammates. He can put this on multiple targets. On top of that, his ult is an AoE knock-up which works fantastic as both an engage and a disengage. He can destroy a projectile with his shield which cancels out certain abilities. Finally, he can jump to, and increase the stats of, one of his teammates, which is great for preventing assassins from instantly deleting one of his squishies.

Tahm Kench - There's a really good reason for not wanting this one on the rift, he can negate the effects of any crowd control. If his adc is stunned, he just eats them and walks them out of the fight. If his mid-laner is hard focused, he just eats them and walks them out of the fight. This is incredibly powerful in a meta where there are typically 2 primary sources of damage and every other member is either utility or tank (or both). On top of this ability, he can stun, eat enemies and walk them into the middle of the team, he has Twisted Fate's ult and can bring someone along for the ride, and last but certainly not least, the grey health mechanic he has makes him incredibly difficult to kill.

Janna - As mentioned in another answer, no poke comp is complete without her. Her tornado prevents enemies from escaping or chasing. Her slow is incredibly effective and long-lasting. Her ult is a massive heal that can nearly reset a fight and wreck an enemy team's positioning. She also has one of the biggest shields in the game, and has the only targeted AD buff in the game, which can sway a lane match-up significantly.

Morgana - There's really only ever one reason to get Morgana. Black Shield. This ability makes its target immune to crowd control. That means, no stuns, no knock-ups, no hooks, no fears, no getting eaten by Tahm, nothing. This ability means your ally has the opportunity to soak up a bunch of very critical abilities that your opponents are saving to control a team fight. Without those abilities, your opponents may lose a major advantage that they were relying on.

Why Not Leona?

Leona offers just two things. A bunch of crowd control, and the ability to not die for a long time. Both of those things are features of the champions discussed above. However, the champions above offer much more than just these features. That doesn't mean that Leona is bad, it just means that she doesn't offer anything special in comparison to the other champions discussed.

But, Leona is so Good!

One of the main reasons Leona is so fantastic, especially in lower Elo matches, is because of her consistency and natural resilience. She has some of the highest base defensive stats in the game, and some of the best defensive stat scaling. Coupled with her shield and her vast amount of crowd control, she is a fearsome bully in lane.

Leona is an effective champion, there is no doubt. However, in the context of the current meta, taking into consideration coordinated team play, and a much deeper understanding of team composition; at the pro level, there is no place for her.

Hate being generic. But solo q is different than tournament. A few things to think about: does leona fit into the team comp they want? Im sure there are tons of viable team comps with leona. She is kind of risky. Yes she does lots of damage high cc. But she HAS to be in the middle of everything to do well. After she throws down her ult and q the rest of the team can ignore her and run to the back line. I like leona as well but its not simply pick what can carry you to gold.

I feel like this response is useless but it might give some insight.