Mr Jack said relentless attacks had destroyed any opportunity for him in politics and his stint lasted just 28 days.
"These attacks have been careless, orchestrated, out of context and demonstrably inaccurate," he said.
"Comprehension of satire has been traded for woke stupidity.
"My kids said I’d be cancelled. They were right."
Mr Jack had been on thin ice after news reporting this week highlighted he had shared a video in 2020 that included a joke about young women.
His candidacy was plunged into further trouble on Wednesday when Stuff reported he had shared a poem on Facebook in 2021 that compared former prime minister Jacinda Ardern with Adolf Hitler.
By the end of the evening, Mr Jack had resigned and National Party leader Christopher Luxon was yesterday pleased he was gone.
Mr Luxon called content shared by Mr Jack crass, offensive and sexist.
National MP Erica Stanford said sharing the poem about Ms Ardern was unacceptable and disrespectful.
"No matter what you think of Jacinda Ardern, she was our prime minister, and she deserved some respect."
Mr Jack described media coverage as character assassination and said the way comments — "which were never mine" — had been presented, or misconstrued, was vile and offensive to him.
"They are incredibly damaging and revealing of the world of modern politics that I have decided I want no future part of."
The National Party has had a string of problems in selecting candidates in recent years.
Mr Luxon earlier this week defended the party’s candidate selection process, saying he was proud of what had been put in place since he became leader, including working with the party president to ensure there was "a very good vetting process".
National Party president Sylvia Wood said the party had worked hard on its selection processes.
"This election we will have close to 40 new candidates, and to date we have vetted almost 150 nominees over hundreds of hours," Ms Wood said.
"The reality is we won’t find everything there is to find about everyone who wants to stand for us.
"Candidates must also take responsibility for themselves.
"They are clearly told that social media can pose a problem if content doesn’t represent the party’s values and standards or has the potential to distract from the very real issues facing Kiwis right now."
After vetting, candidates went through a local democratic selection process.
"Most of the time, local members get it right, but just like any selection panel, sometimes they don’t.
"At the end of the day, Stephen Jack has resigned and that is the right decision."
A new candidate for the Taieri electorate could be selected by the end of May.
Mr Jack is based in Balclutha and has been farming his family’s sheep and beef farm in Hillend since 2005.
Federated Farmers Otago president Mark Patterson, a former New Zealand First MP, said Mr Jack was a good community and family man and sometimes presented strong opinions.
"He does shoot from the hip, but he is willing to listen to other points of view."
Mr Patterson suggested question marks remained about National’s processes.
"Jacko’s Jacko. You know what you’re getting."
Former Federated Farmers Otago president Simon Davies had concerns about media treatment of some political figures and indicated he preferred Mr Jack to a candidate who was "perfect, presents well in front of TV cameras and wears the right shirt".
Neither man endorsed Mr Jack’s social media activity.
University of Otago politics lecturer Dr Chris Rudd said parties had to be frank with candidates about their social media activity being considered fair game.
He expected Mr Jack’s experience would add to some reluctance potentially good candidates might have for putting their names forward.
Politics professor Janine Hayward, also from University of Otago, said social media was a relatively new way for people to gather information about candidates, but parties had always needed to be satisfied their candidates were a good match with party values.
Candidates could turn into a liability.
In the past and present, the lives people had lived could present challenges for parties selecting candidates, Prof Hayward said.
"Your past can catch up to you as a candidate."
