Why is he still in the game when Kalista does everything better than him?
Or, or we can ask ourselves why Twitch is untouched while he's possibly the weakest ADC and why Kalista is untouched while possibly being the strongest ADC.
Oddly enough I've had more success with Twitch in ranked than Kalista, mostly due to Kalista being very support dependent on L6 trades, whereas Twitch can just sit back, pop his ult and mow.
probably because twitch's late game scaling is way better. he can invis for 8 seconds, 8
then when you're in range, pop your ult, use your full damage build and proceed to kill everyone but the fighter/tank, or if their teamcomp is mostly tanky, you'll probably kill them anyway
Twitch has higher damage that's more conditional, Kalista has lower damage that's less conditional. In other words, Twitch does better in short skirmishes and Kalista does better in long, drawn out fights.
Neither are the strongest or weakest ADCs, which have high AND unconditional damage
or low AND conditional damage 
.
Kalista isnt the strongest ADC, Twitch is just below average right now. His passive could use a buff late game and his W could use some tweaks to make it even more useful since it isn't a big WOW spell or even an average OK spell.
why would you even compare twitch to kalista? Twitch can assassinate a whole team if used right, Kalista is just a freakaccident that jumps around and annoys anyone to slow and horrible deaths. Twitch is still the ADC with the most potential by far, he´s just way harder to use/easier to play around than Kalista.
The problem with Twitch is that his core items are BotRK and PD which take him a while to get, and have the ball rolling, and his E and Passive are both extremely UP. The 6 stack E does about as much as 1 AA. Which means by 6 AA's they're usually dead, and don't even get me started on the poison. The only thing it does is stop health regen for a couple of seconds.
Yeah twitch is a pretty easy to play champ that has tons of power in his kit, while not the best at anything he is a very safe pick.
Really what has changed on him since S4 worlds, where he was a hotly contended pick?
