There is a quite old thread on Adobe forums where speculation is made that it's because Fireworks is a screen-specific tool and this behavior might change in the future.
However, you can change the limit.
Make sure that Fireworks is closed, open
Fireworks CS6 Preferences.txt file located in:
- Windows 7/Vista:
- Windows XP:
C:\Documents and Settings\USERNAME\AppData\Roaming\Adobe\Fireworks CS6\English\
- MacOS X:
Library/Preferences/Adobe Fireworks CS6/en/
or adjust this number as needed.