First of all because anybody can read the contents of files there - presuming you don't have a sane umask.

Secondly because other uesrs might create files. Imagine somebody ran:

     ln -s /home/foo/.bash_profile /tmp/clipboard-data.txt

The next time you ran your clipboard-script your .bash_profile file would get its contents erased, due to the symlink.

Predictable filenames are a big security hole for multiuser systems, even though in practice you're probably the sole user of your desktop/laptop. I've reported numerous bugs of this kind, including this small set against GNU emacs:

http://seclists.org/oss-sec/2014/q2/269

Someone could put a symlink at that location to direct the output to clobber or create a file under your control.

Probably because everyone can access your clipboard