It is because tuxguitar is a "tablature" editor. and as it, you can't insert more notes than strings you have, as you can't add notes out of tunings.
if you want to simulate a real piano track, you'll need to use 2 tracks (one for treeble and other for bass)
and set the track tunings as you need.
however i think that you need is a real score editor instead of tablature editor. i suggest you rosegarden for gnu/linux.