"Bad writing", anything I don't like. Honestly, 'bad writing' lost all meaning.
I'll be blunt, both Trunks and Goten are terrible fighters. They have a lot of promised and are strong, but in terms of actual fighting skills they are the weakest. They're reckless, childish, stupid, and would have been a horrible choice for this type of tournament. It would've been 'bad writing' to include them since they're completely undependable. Yamcha would have been a better choice then both of them.
The Z-Fighters put their fate in them to stop Buu and look how they royally fucked that up. Why would the depend of them again when the universe could be erased? If anything, it shows the character have been getting smarter by not using them since they haven't shown any mature growth as fighters and not just looking at power, which includes Gotenks who is Goten and Trunks' reckless and childish behavior times one-hundred.