Think of it another way...
You don't see Ford putting GM engines in their new cars. They have their own engines and want to use those in preference, because it is a pride thing. To use another manufacturers engine is to admit engineering defeat. (Although there are plenty of example of co-operation between manufacturers to save on R&D costs by sharing engines and components (see Peugeot/Citroen and Toyota.)
The same goes for Samsung and they have used their own processors in each generation of the Galaxy S line, but not in every model, for every market.
There were some combinations of processor and LTE that didn't work and they had to use Snapdragons in those markets. Maybe they have now solved the compatibility problem and are keen to show off their own processor.