Because it's a different filesystem (VFAT) that does not support all the Unix file permissions like the "memory for installable applications" (/opt - ext3 IIRC) does. Another reason is that whenever you plug your N900 into your computer using USB Mass Storage Mode, the 23.03 GB partition (aka MyDocs) gets unmounted and "exported" to the PC. If you have an application running on your N900 that's using something from MyDocs (and if the application itself is put on the 23.03 GB, it counts as using too), this will prevent the partition from being exported to your PC. Also, if you manage to connect the N900 to the PC, and the export works, you can't start any application on that partition from the N900 (because the N900 does not "see" it - it has been unmounted and exported to the PC).

What you could do is resize the /opt partition and MyDocs (i.e. make the "23.03 GB" MyDocs partition smaller and reserve more for "/opt"), but that's an advanced topic and not something that should be done without taking backups first and risking a reflash There are some threads that describe how to do this. But again - if you want to go that route, make sure that you backup everything and be ready for a reflash should anything bad happen.