Why can't 'data' folder be displayed just like in DDMS file explorer?

I list out files in '/' by:

File directory = new File("/");
fill_listview(directory.listFiles());

And I get those in list:

sqlite_stmt_journals
config
cache
sdcard
d
etc
system
sys
sbin
proc
init.rc
init.goldfish.rc
init
default.prop
data
root
dev

I have two questions:

1.Why cann't I access '/data' folder just like Eclipse DDMS file explorer?

2.Why it is different from the DDMS which show only:

data
sdcard
system

Thanks!