The medicament Chlorprothixene has been around since 1959. Why is it not approved for use in the United States?
Probably because it was only available from one company (Taractan from Roche) and it was no longer successful on the market, so the company discontinued it, apparently in 1997:
Hoffman-LaRoche, Inc., et al.; Withdrawal of Approval of 11 New Drug Applications [...]
EFFECTIVE DATE: February 10, 1997.
SUPPLEMENTARY INFORMATION: The holders of the applications listed in the table in this document have informed FDA that these drug products are no longer marketed and have requested that FDA withdraw approval of the applications. The applicants have also, by their request, waived their opportunity for a hearing.
NDA 12-486...................... Taractan Do.
(Also in pdf should you care).
And no other company in the US seems to have found it worthwhile to apply for authorization for a drug with the same active compound. (This isn't all that unusual, by the way.)
You can actually send an inquiry to the FDA which forces them to say whether the drug was withdrawn for either safety of efficacy reasons. I haven't found one published for TARACTAN, which basically means nobody cared to inquire.
Note that by 1997 2nd gen ("atypical") antipsychotics were starting to eclipse the 1st gen ones on the US market. (Ten years later, in 2008, they were 90% of the US market. This was not the case in Europe, where that fraction was only 40% in France in the same year.)
Taractan received its FDA authorization sometime in the early 1970s or so (there are several notices some with changes in recommendation from that era, and I'm not inclined to read all of them.)