This is because the strings... it's a tablature software so you can't add more notes than strings you have, as you can't add lower notes than the low string is tuned.
So you could change the tuning of the track to allow lower notes, and maybe is better to use 2 tracks ( left + right hands )
BTW, if you are a piano player, tuxguitar isn't your best option.. i suggest you rosegarden instead.