Well part of it is probably that TV shows need to be either inverse telecined or bobbed for deinterlacing. This process doesn't always work out very well and you can see stutters or weaving (interlacing) in the result. And of course the Trek DVDs, TNG-VOY in particular, are pretty grainy and dirty transfers. Upscaling is a pretty nasty process overall too. It's impossible to recreate picture detail that simply isn't there, and you're also enlarging the grain/noise.