Why are ARM9 and ARM11 processors still used when the Cortex-A5 is more efficient?
Migrating an application from one processor family to another requires lots of planning and design changes. That in turn equates to a certain cost to migrate.

Following are a few factors involving costs:


Physical redesign
Code redesign
Testing
Skills acquisition 


ARM has a roadmap for migrating from ARM9 to Cortex.

In a nutshell, cost is the biggest reason why manufacturers stick to older designs.