1. Need more senior resource -- they want somebody stronger (in their eyes, you may have different opinion). They can make it more appropriate, like saying: your qualifications are impressive, however, they're looking for somebody that provides a closer fit.
2. You addressed well most of the inquries in interview -- which means you missed some, and that could be the reason you did not meet the requirements.
3. "Senior" in the past does not matter, we're in an competitive environment, everything in the past is over.