Actually both can store between 4 and 6 application at the same time.
The nano s has 320 kb - the ledger blue 480 kb.
The main idea behind this small storage is to limit the amound of code 'which could be injected'. Its a security 'feature'.
As far as i know the ledger team is working on an app handling all currencies. But didn't found anything regarding this on their roadmap.