Agree with Kacee.
I know there are some that want to believe that BKK was higher yielding than many others suspect, but there is a reason that when UA started new ULH flights to South Asia last year, it was to SIN and not BKK, and two flights to boot. When LA to SIN didn't work out, rather then use that aircraft to do BKK, they simply kept the second SIN frequency to SFO.
And UA wasn't the only US carrier that decided BKK wasn't worth it with their own metal - DL followed UA out not too long after. UA still has JV operations via NH at NRT at least, and I'd bet for UA, it's a more efficient way to serve BKK.