I'd have to guess it's a combination of population and history. The Empire is much much younger than the Republic or Nar Shaddaa even this early in SW history. It's also more remote, being based on the rim as opposed to in the core. I don't think the Empire has as many people in it (even including the slave population) as the Republic as a whole. Dromund Kaas could potentially develop that way given enough time.
Then there's the whole Sith infighting thing--their paranoia and constant destroying each other isn't conducive to extensive urban development.
I was thinking more along the lines of species diversity and dense population of aliens that are not slaves. It's true given the desire and enough slaves Dromund Kaas could eventually become an ecumenopolis (I learned that word from you) though the weather and much larger surface area might also be a challenge for that particular planet.