Well, because of the way the Partition bullet is designed to work! It is DESIGNED for violent, rapid, explosive expansion of the front core (depending on how fast it is going when it hits), followed by deep penetration of the rear portion. The partition bullet will expand reliably even at long ranges of 500+ yards, after the bullet has lost a lot of velocity, yet it holds together right off the muzzle, even at magnum velocities. IF Nosler bonded the core for great weight retention, they'd lose the violent expansion/long-range reliability they now have, and the resulting bullet would not be as good as the ones they are now making. If you want bonded, shoot the Accubonds. They have a higher B.C. as well as a bonded core.