Different chipsets have different features, intergrated video, multi gpu support all that stuff. Depending on what brand of video card you chose, Nvdia for SLI support and ATI for crossfire support. Some users will buy a card now and get a new one to add on when they need the extra performance. Most people don't need the multi gpu support so having another PCI-Express slot won't effect them.
As for the manufacturers, some manufactuers have a great reputation like XFX and evga that make reliable cards or factory overclocked cards that work well. Other brands like foxxcon, PNY, foxxconn are known for cheaper but unrealiable products.
Before all those brands just made motherboards like msi, gigabyte and asus. I was pretty surprised at first when some other brand than sapphire was making videocards for ati and some other brand than BFG was making video cards for nvdia
just cuz its in stock now.
