1. Haven't ridden one, so I can't comment on durability. The mountain bike market is so saturated that sometimes companies come out with unique designs just to seperate themselves from the crowd.
2. Preference. Some companies claim that their design is more efficient that others, but no one has built a suspension dyno to accurately measure the power lost between the cranks and rear wheel during suspension travel. Less pivots = less maintenace.
3. Avid, Shimano, Hope, Hayes, Formula and Magura are the major players.
4. Preferance. Shimano dominated the '90s, but Sram has totally caught up and in some ways surpassed them.