Chairman Huw Jenkins says the lure of the Premier League was too much for Swansea City manager Roberto Martinez to turn down.
Martinez is set to complete his move to take over as Wigan boss on a three-year deal on Wednesday.
But Jenkins told BBC Sport Wales he was "bitterly disappointed" by the Spaniard's imminent departure.
"He wanted to take up the opportunity of testing himself against what the Premier League had to offer," he said.
"That's what he based his decision on in the end.
"He's a very ambitious man. Sometimes when you take on ambitious managers, that type of thing does come.
"They are going to be eager to move on, eager to develop their career and sometimes that's the nature of the beast."
Jenkins admitted he had offered 35-year-old Martinez a pay rise to stay which was turned down, but said that the pair parted company on good terms.
"We've got a great relationship that we've had not only for the last two-and-a-half years he's been here as a manager but as a player.
"I think when we get to these situations you've got to take into account the personal feelings of who you are dealing with, whether it is a player or manager, and you've got to deal with that accordingly.
"You can be assured our relationship will be intact after this."
Jenkins dismissed speculation that Martinez had become frustrated with dealings in the transfer market.
He also defended the Spaniard over declarations of loyalty Martinez has made in the past stating that he would have to be "forced out" of the Liberty Stadium.
"I think he certainly made those statements," added Jenkins.
"You mustn't forget he's a young man learning his way in the game and perhaps looking back or in the future he may be a bit more careful about how he says things.
"But I'm sure he did say them in good faith from his heart as he felt at the time."
Jenkins would not divulge how much compensation Swansea would get from Wigan, but it has been reported to be between £1m and £2m.
But he said no amount of money is not going to be "a substitute for losing Roberto".
"I'm bitterly disappointed," Jenkins added.
"I felt that after last season just missing out on the play-offs we would bring a few players in, adjust a little bit and really look forward to another challenge to see if we can get into the play-offs at the very least.
"I think all our supporters were fully looking forward to that. It's obviously a big blow to us.
"I thought when he signed his new contract with us, he would be here at least three to five years and we could see where the club had gone over that period, and whether he stayed with us longer would be down to his wishes.
"That's how I felt and how I thought was going to be the case."
Jenkins issued a rallying cry to disappointed Swansea supporters that the club has to move on from this.
He said: "The club is greater than any man and I just want everybody to get fully behind whoever is in charge, but most importantly fully behind the club 100% and keep us going forward."
No leading candidate to take over as Swansea manager has yet emerged, with only Falkirk boss John Hughes and Bury's Alan Knill publicly expressing an interest.