You should have been given a report card when you were banned. If not, check your email and try logging into the client. It should be there.
As for why you didn't get a lesser punishment, punishments are dished out on an escalating basis generally. Meaning if a lower-tier punishment did not work, you will be escalated to a higher tier, including up to permanent suspension. For example, say I insult my teammates (no hate speech or anything, but toxic) and get a chat restriction. I do the same the again I will get banned. I do it against I get banned for a longer period of time. I do it again I can get a permaban. Why? Because no reform was shown after the warnings, so for the good of the community I would need to be culled.
Now, really bad offenses like hate speech, or telling others to kill themselves, may result in higher punishment faster and skin the tier system. But for most instances, punishes get progressively worse, with the 14-day ban being the final warning before perma.
If you post chat logs, I could help you go over them if you like