why did i get 15 day ban for trash talking.

Why did you flame?

why did i get 15 day ban for trash talking.

why did i get ban if i could get just in game mute.

Well, there are two highly probable ways you could've come to the 14-day Ban;

1. Standard escalation. If you've recently come off of a 25-game Chat Restriction, then the 14-day ban would be the next step. The punishment ladder is as follows; 10-game Chat Restriction > 25-game Chat Restriction > 14-day ban > permanent ban. Continued misbehavior results in harsher and harsher punishments. They aren't gonna give you infinite chances to keep misbehaving the same way, after all.

2. Zero-Tolerance escalation. Alternatively, if you jumped straight to the 14-day ban without a prior punishment history, it's very likely you used zero-tolerance chat, like hate speech or encouragements of self-harm. You only get one chance to reform from such misbehavior, as Riot expects you to already know that such behavior is unacceptable.

Whichever way you arrived at the 14-day ban, a chat mute/restriction would not suffice for a punishment. Your behavior was either egregious enough or frequent enough to warrant a temporary suspension - which, might I add, is stated on the reform card to be your last chance. Any further misbehavior will result in a permanent suspension.

If you want to have your behavior peer reviewed, you'll have to provide your chat logs, elsewise, this is about as much information as I can feasibly provide.