Party leadership knows Trump is bad for them, so they will not support him.
Rank and file appear to have more Trump supporters than Harris (or Biden prior to the switch.)
Therefore the teamsters feel that they're screwed if they endorse either one.
The article is worth a read.
Because the Teamsters, like many blue collar workers, are racist and sexist Trump supporters. It really doesn't need to go farther than that.
https://www.theguardian.com/us-news/2024/jan/31/teamsters-racial-discrimination-lawsuit
https://finance.yahoo.com/news/teamsters-hurled-sexist-anti-muslim-154544490.html