All tracks but one have been issued on Blue Note and Pacific Jazz Art Pepper CDs before, including the Joe Morello and Bill Perkins dates, but the CDs are now all OOP and appear to be pretty expensive. The Joe Morello tracks appeared on the Blue Note CD The Return of Art Pepper, the Bill Perkins tracks on the Pacific Jazz CD The Artistry of Pepper. The track missing on the individual CDs is "Angel Eyes".
As DrJ said the Pepper Mosaic was 24-bit remastered, which people may prefer. Also, a Mosaic set has a special appeal for many people, especially when it's OOP.