Why was the tribunal removed?

Because League has a very toxic community and a lot of people were choosing the "punish" option without even reading. Imagine thousands of trolls being able to review your case and 90% of them condemning you when you didn't do anything wrong, i'm glad this system was scrapped.

Because people report for the most petty, unimportant bullshit. With such a large playerbase, this means the number of reports is astronomical. Having a tribunal of trusted people go through these reports just takes too long. You may think, just have more people join the tribunal to go through the reports, but then you open it up to trolls joining and punishing people dishonestly.

Simple.. the community is to large to not have an automated system. Its not like csgo that has 40k people at a time maybe. There are millions playing at one time and its not possible to have humans go through all of the reports

It was only used for edge cases, it took too long.

It took too long to punish people. Riot traded accuracy for speed.