Not saying that wasn't ONE of the reasons but the main reason was really that a large chunk of users had no idea that they even had a discriminator, as it was added on top of your chosen username. "add me on discord, my username is slashink" didn't work as people expected and caused more confusion than it was solving. This wasn't universally true either, if you come from a platform like Blizzard's Battle.net that has had discriminators since Battlenet 2.0 came out in 2009 it was a natural part of your identity. End of the day there were more users that expected usernames to be unique the way they are today than expected discriminators.
Addressing that tension was the core reason we made this change. We are almost 3 years past this decision ( https://discord.com/blog/usernames ) and I personally think this change was a positive one.