You also have to consider that many condos are purchased with a mortgage, no different than many other countries. With a new condo the developer has a relationship with the bank who is financing the entire condominium project (and often other banks as well), so minor issues between the customer and the bank can often be smoothed over.
Someone buying a used condo unit will have to deal with the bank entirely on their own, and that is just not as easy. For example you will need the unit surveyed to assess its value, whereas with buying a new unit the bank has already assessed every unit in the building, saving cost, time and allowing the bank to offer a "package" type mortgage.
It's often not that people don't like used units, it's simply a case of finance. I guarantee that if I was selling 10 used units on Sukhumvit or Sathorn at an 80 per cent discount there would be a queue around the block.