Thailand is much better than many other countries for UK visitors, its USP is not its sex industry, there are many factors.
Simple logistics for example. From London you can fly direct to KL, Bangkok or Singapore in that region, and not to other countries.
Singapore is ruinously expensive, and a tiny country with only about three days worth of attractions. Malaysia is rather prim, being Islamic.
Bangkok is closest of all those cities, and the flights tend to be a little cheaper for that reason (also competition).
The Philippines is a nice tourist destination from what I understand, but I've never been there because the extra cost and time in travelling there, it involves typically three flights rather than one to get to somewhere with a tourist industry, whereas I've been to Thailand half-a-dozen times.
Also food in Thailand is some of the best in the world.
You could stay in a world-class 5* resort such as the JW Marriot Phuket for £70/night including breakfast, which in the UK would buy you a night in a Travelodge.
Infrastructure is a big factor. It can take a day of travel in a shitty bus to travel between places in many countries in the region, Thailand is sufficiently developed to make it attractive.
Unlike visiting India you don't have to pay £100 and drag yourself to sit in shitty office in Victoria and wait in a long queue - you just land in Thailand, get your passport stamped, and you're done.
If you look at possible tourist destinations:
USA - expensive
Caribbean - expensive, small countries making transport expensive
South America - expensive flights, crime problems
Dubai - built on slavery, expensive
Europe - expensive, limited season, not exotic
Gambia - limited season, small country
Southern Africa - wildlife, but no beaches, crime problems, can be expensive
India - visa bullshit
Morocco, Tunisia, Egypt - hassle from locals (IME)
Sri Lanka - small country
Australia - expensive, ridiculously long flights
Indonesia, Cambodia, Vietnam, etc. - best accessed via Thailand anyway
China - visa issues, very large
There aren't many places where you can fly in on a whim from London, stay in a 5* hotel for under £100/night, eat good food for £2, visit golden temples and then be on the beach the next day or in the jungle with elephants, etc.