Philadelphia will not host Forbes' 30 Under 30 millennial-fest this year, and probably not any time soon.

The organizers announced Monday - at a news conference attended by the mayor of Boston and the governor of Massachusetts - that the four-day gathering will head to Boston in October.

"I first want to thank Philadelphia for serving as host city over the last two years to kick off our Under 30 Summits," Randall Lane, editor of Forbes magazine, said in a news release. He went on to say that Boston's summit would triple in size and include young entrepreneurs worldwide.

Forbes hopes to keep the summit in Bost on indefinitely, Lane told the Boston Globe.

"If Boston supports this as we think they will, we have no desire or interest in going anywhere else," he said. "We like the idea of being a catalyst for the city and burnishing the city, as the city helps us."

The move comes just more than a year after officials called Philadelphia the permanent venue for the invitation-only millennial gathering, which was held at the Convention Center in 2014 and 2015 and drew high-profile figures such as Monica Lewinsky and Malala Yousafzai, the young Nobel Peace Prize recipient.

Former Mayor Michael Nutter spent $1.7 million of the city's money to bring the 30 Under 30 Summit to Philadelphia two years in a row.

This year, Forbes asked Mayor Kenney to commit $2 million in private sponsorship to the event.

Kenney said he asked Forbes to provide an invoice of expected expenses and details of the economic impact of the conference on the city, but received neither.

"We think it's a worthwhile event and we're sorry we lost it," Kenney said. "We were committed to $2 million, some city and some private, but we needed a precise list of their expenses and that was not forthcoming, and I guess they were pursuing other opportunities and went with Boston."

Forbes organizers said they went out of their way to try and meet with Kenney to keep the conference in Philadelphia. After a phone call and sending in a proposal, they "heard nothing," said one person close to the negotiations.

In its proposal to the city, dated Jan. 29, Forbes 30 Under 30 organizers wrote they were frustrated with the city's response.

"While we certainly recognize that a mayoral transition is an inherently hectic time, the lack of formal clarity about Philly's interest in continuing in the role of host has left us no choice but to begin to explore other locations," Forbes wrote.

In the proposal, Forbes expressed interest in keeping the event in Philadelphia and using private money to fund it. Nutter faced criticism for using public money in the second year.

"Be assured that we never have pursued a taxpayer subsidy for this event, nor are we interested in that going forward," the proposal reads.

Lane, a graduate of the University of Pennsylvania, declined to comment on the change of venue but said no city money in Boston will be used for this year's event.

Lauren Hitt, spokeswoman for Kenney, said the city didn't want taxpayers on the hook if private money didn't come through.

She said the fund-raising potential looked difficult, given "that a lot of the donors who would normally fund something like this are already getting tapped for the [Democratic National] Convention this year."

The Philadelphia Chamber of Commerce said Kenney's administration did not reach out to it about the conference.

Ed Grose, executive director of the Greater Philadelphia Hotel Association, said that while the event could attract attention to the city, the clientele isn't quite as lucrative for business as, say, a life sciences convention.

"I think the group had a lot of panache, it's a well-known group, but you have to look at the return on that investment, and I'd imagine $2 million could buy a pretty big convention," Grose said.

"Look at a bio or medical convention - you not only have doctors but pharmaceutical sales people, and they're more likely to go out to our nice restaurants, spend more entertaining. So it's not just more nights in hotel rooms, but people who are spending more."

Comcast Corp. executive David L. Cohen, former chief of staff under Mayor Ed Rendell, agreed with Grose. The event, he said, "under-delivered on its potential."

"We were a sponsor of this event for both years - including providing significant media value through MSNBC. We honestly never saw the value we expected from the conference," Cohen said.

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Staff writer Emily Babay contributed to this article.