Originally Posted by Karina
I think the only reason is that many devices do not have much space left on their internal storage.
Noted. I wondered if it was for compatibility issues since there is an "Automatic" option (in the codes, that's) which lets the system decides where to install (internal or external), and nowadays many devices have lots of internal space, for instance, on mine there's 1GB of free space on internal.
One potential downside of installing to external is that the app wouldn't be available when the device is connected to a PC/Mac and have the external storage mounted.
Not something particular through.