Good question! Found this, should explain things:
President Trump finding it difficult to endorse a candidate who called him charlatan who is unfit to be President, supported by a cast of characters who said Trump supporters should be thrown over the wall.
All-out warfare has been raging in the New Jersey Republican party for over a year and three camps of Republicans are battling it out in the streets to be the ones to challenged Governor Phil Murphy in November.
First, you have the establishment Republican candidate Jack Ciattarelli who has the part (swamp) line in every county statewide. You would think this endorsement by GOP leadership would be a no-brainer for President Donald J. Trump’s endorsement, yet here we are, three weeks before one of the most critical primaries in the United States in 2021 and Trump has been silent.
Trump will soon be returning to New Jersey, but he’s not likely to get involved in the New Jersey Republican primary as he has done throughout the country on less important elections.
Why? Trump’s in a bad spot in New Jersey. He cannot back Ciattarelli openly because it would send the wrong message to other Republicans who have defied the former president. Ciattarelli was a well-known “anti-Trump” establishment guy for a long time.
Ahead of Trump’s 2016 election win, Ciattarelli called the President a “Charlatan” who is “Unfit to be president”.
Those words are now biting him as Trump has yet to give the establishment swamp guy his endorsement. If Trump endorses Ciattarelli it would send the message nationwide to the never-Trump crowd that the former President is willing to forgive and forget the actions of his enemies and detractors.
https://www.shorenewsnetwork.com/2021/05/17/heres-why-president-trump-has-not-endorsed-nj-gop-candidate-jack-ciattarelli-for-governor/