Thai Airways used to fly nonstop from Bangkok to New York and Los Angeles using their Airbus A340-500 aircraft, though in 2012 they cut those routes, given how much money the airline was losing on the routes. This was due to a combination of the A340-500 being a gas guzzler, and also the generally low yields on flights to and from Thailand.
Thai Airways used to operate a one stop flight between Los Angeles and Bangkok, which initially routed via Osaka, and then eventually via Seoul Incheon, before it finally got cut. When an airline only operates a one-stop routing to their home country, they don’t really have a competitive advantage, since there are a dozen airlines operating one-stop flights between the US and Thailand.
Thai Airways isn’t even allowed to fly to the US at the moment, given the poor rating Thailand scored in a safety audit. This caused the FAA to downgrade Thailand to Category II, which means they can’t operate flights to the US. However, I’d keep in mind that’s more a reflection of the regulation in the country overall (or lack thereof), rather than Thai Airways specifically.
Anyway, it looks like Thai Airways is looking to change their safety image, and even wants to restart flights between Bangkok and the US. Per the Bangkok Post: