Slack interview questions

Applied online on their website. Recruited emailed showing interest. Scheduled the call for initial screening. Recruiter asked my background and explained the process. Surprisingly got a reject email right away. Not sure what happened. If profile did not match then what was the need for the screening call?

Email received by HR Recruiter asking that I complete a written assignment within 24 hours. Next came the phone interview by an amazing lady in the HR department. She was really impressed with my skill set and the completed written assignment. She moved me on to one of the Customer Success Managers. The phone interview was fine, he even explained the next process but I felt others were listening as I heard people speaking in background. I didn't move on from there and could not get an answer as to why. I have a great customer service and tech background. They admitted that many of their agents have less experience than myself. Perhaps they are only looking for Millenials, not sure! :(