My personal views, and I'm a bit of a food snob.
I either want a good fish and chip shop, or a good Thai shop. Not both. A dual Chinese/fish & chip shop usually means cheap, bad quality food in my experience.
I don't know why there aren't many fast food Thai places, I think that Thai food is a lot more refined than many of the Chinese takeaway places. (Indeed most proper Chinese restaurants are a lot more refined that most Chinese takeaways)
Also many ingredients in Thai food are more expensive so takeaways will be more expensive.