When Horner did the score to TWOK and TSFS, he wasn't as well know as he was when TUC was made. So like Goldsmith, Horner was out of the film's price range for scoring TUC.
Based on his latter interviews I think Horner never really respected Trek fully, but saw it as merely a learning experience and stepping-stone, like the material was somehow beneath him. Goldsmith I think began to see his music's relationship to Trek as equivalent to Williams with Star Wars.