Offensive tackle Levi Brown is probably not good enough to be protecting Manning or any quarterback, and definitely isn't the guy you want to rely on in the long-term. Adam Schefter of ESPN filed this report.
Cardinals are expected to release OT Levi Brown, whom Arizona selected with the fifth pick in the 2007 draft ahead of RB Adrian Peterson.— Adam Schefter (@AdamSchefter) March 12, 2012
The contract restructuring was probably the main issue, as the Cardinals probably weren't willing to put that much money into him. This could be a good thing for Brown, because with the lack of strong offensive tackles in free agency, he could fetch a strong deal from a team in need. Brown has had his issues in pass protection though and has only played well in spurts, so it makes sense for the Cardinals to let him go.
To talk about how Arizona should replace Brown from Cardinals fans, check out Revenge of the Birds.