Benitez enjoys cult hero status at Newcastle thanks to his time in charge, which included leading the club back into the Premier League.
However, he admits he chose to leave as he couldn't see a project forming under former owner Ashley.
He told ChronicleLive: "I was very close to staying.
"But, with the situation when I was leaving, Mike Ashley was not selling at this time and we could not wait. It was not a proper project at this time because there was not any intention to invest, to improve things, so we had to do it.
"The timing was a pity, but it was what it was. I couldn't wait because it was very clear he had no intention to invest and to improve so we couldn't be waiting and waiting."
Reflecting ahead of Sunday's Carabao Cup final between Newcastle and Liverpool, Benitez continued: "We experienced some years of suffering without any chance to make real progress.
"Now they have had this possibility in the last few years and they are growing and challenging for trophies, which is amazing.
"It's been so many years. It's been a sleeping giant. It was a pity that they could not be at the level that the fans deserved, but now they are there so I'm really pleased for them, for sure.
"In modern football, when you have some money and you can invest, you can improve. You have to compete with clubs with a lot of money and if you can't get the best players, it's very difficult to have a chance to win a trophy.
"But Newcastle United have improved a lot in terms of ownership, investment and staff in the last few years so everyone is doing a good job and the fans are delighted. I think they deserve to win but I cannot say, 'They must do it!' because then the Liverpool fans will be like, 'Oh, Rafa!' So I'm happy to see both teams there."