I don't think some storylines require or are worth
the time and effort of creating radically different and unique aliens.
A lot of the time a generic 'forehead of the week' design will suffice.
Inevitably when faced with seven years' worth of creating new designs for alien races some will be more radical than others - for every Species 8472, Hirogen or Vidiian you'll get half a dozen Taresians or Krenim-type aliens.
I wouldn't call it laziness as much as the reality of weekly television production.