Sex tourism
Not just that. The seedier stuff is there but is confined to one area of the city.
Bangkok is a great place to visit. There is a lot of English spoken. There are lots of interesting sights, shopping and places to eat. It is also fairly safe without a lot of violent crime. It was, at least at the time, also cheap compared to HK.
When we lived in Hong Kong we used to take the kids to Bangkok or Phuket once or twice a year for a weekend get-a-way