Originally, there was no National Master. Small Locals sometimes joined together and negotiated as a group. In 1979 many of these regional groups were brought together under the first National Master Agreement. The Master was a limited document, as it is still today, and each region kept its own agreements (now called Supplements).
Some Locals are so large they are as big or bigger than other regions. Locals 705, 710 and 804 for example. Each Local or regional group must decide if it is in their best interest to join the National Master. Unfortunately for the Master, it doesn't have much to recommend itself. Much of what it covers is already covered in the Supplements (or the independant contracts of 705 and 710). There's the huge part-time concessionary language, the huge Air Operation concessionary language, and the problem that the Locals get lost in the mix when the International takes over.
Some may see the International as being in bed with management and do not want to place themselves at the International's mercy.
Note how the International moved up the voting timetable so that nearly everyone was caught off guard. Voters had very little time to digest the proposed agreement. A phoney January 1, 2008 pension deadline was invented. Everyone was told we had to get an early agreement so the customers wouldn't get scared off, and yet here we are with the contract negotiations of Locals 705 and 710 and the Machinists Union obeying the true July 31, 2008 expiration date anyway.
If you read the proposed Master and then saw how the Teamsters officials nationwide voted unanamously to recommend that we accept the contract on the first go-round, why would you want to give up your local control and independence and join?
Why dilute everyone's vote by voting on numerous issues that don't directly effect you and about which you may know very little. My ballot had me voting on employees of CSI, UPS Latin America, Challenge Air Cargo, Trailer Conditioners, Inc., the withdrawal from the Central States Pension Fund and the creation of a new UPS/IBT Pension Plan for full-time Central Staters, the continuation of the UPS Pension Plan (for part-timers in Central States) even though I'm in New England and have nothing directly to do with these and other situations.
The Master also includes each and every Supplement as part of THE CONTRACT even though each region only votes on its own Supplement. You are thus "agreeing" to all the other Supplements without any say, and without even seeing them.
It's all designed to dilute your vote and engineer a ratified contract, no matter how many concessions it includes. Management likes the Master arrangement because they can more easily manipulate the whole process.