Slalom interview questions

My initial interview was with one of the internal recruiters to talk about the company and a high-level overview of my experience. After that, she introduced me to one of the business development employees as well as a client service lead to talk more specifically about how I might fit-in with some of their projects. From there, we discussed specific opportunities and an offer, which i accepted. The whole process took a few weeks but they were always honest and up front with me about the current status and next steps.

Had a phone screen with the recruiter and then later had a 1:1 onsite with the same recruiter, 1:1 interview with a peer to the position, 1:1 interview with someone slightly higher up in the org; and finally a 1:1 with the senior leader of the group I was applying for. The process was fairly routine with half questions about my interests and expectations and the other half was informal behavioral interview questions. I was not extended an offer which was fairly frustrating given the time and energy I put into the process. I was told in my declination email that I didn't have enough experience for the role (I have about 6 years of deep experience and 12 years of other relevant experience). This surprised me given that one of the people that interviewed me, a peer to the position I was applying for, didn't really have any experience doing the work before they hired her. He/she had work experience but none that was related. The employee that referred me to the role seemed really shocked that I didn't get an offer as well given how much experience I had an the glowing reviews he/she gave me based on our previous work together.