Apple had more negotiating power than any other cell phone manufacturer had in the past. The first iPhone that supported the Verizon network was the iPhone 4. At that point it had already been proven to be wildly successful, and it was one of the few things that people could knock Verizon about (aside from price). I'm pretty sure Apple held all the cards in the negotiations to get the iPhone 4 on the Verizon network.
Contrast that with every Android OS based manufacturer:
1) The majority of them were companies that had been working with cellular providers for years already. They probably already had agreements in place with providers about how things would go (bloatware, extensive testing, etc.).
2) They were competing with each other for market share because the OS wasn't a differentiating factor (their bloatware/cosmetic overhauls were weak attempts to establish software differentiation).
3) Verizon made a big investment in Android with the Droid line, providing marketing, branding, etc. Any other manufacturers that were building Android phones had to compete with these devices and had to market on their own. Verizon had all of the cards.
It's slowly changing, but it boils down to:
1) Manufacturers trying to compete (they often add plenty of their own bloatware)
2) Verizon using a model they're familiar with from the feature phone era and manufacturers being used to the carriers having control