Page, 41, left his role at Port Vale on Wednesday to replace Chris Wilder, who joined Sheffield United on 12 May.
"It's all about the chairman, listening to what he wants to do with the football club on and off the pitch," Page told BBC Radio Northampton.
"Having met with the chairman and seen how positive he was, it made it a no-brainer," added the ex-Wales defender.
Page, who guided Port Vale to 18th and 12th during his two seasons in charge at Vale Park, said he was grateful to his former club for the opportunity of his first managerial role.
"It all happened quite quickly," said Page. "I had been at Vale for five years. Micky Adams took me in initially as youth coach and I'll always be grateful to Port Vale giving me my first chance to manage.
"It wasn't an easy decision to leave but it's the next step for me and something I'm really looking forward to."
Ex-Watford and Sheffield United defender Page says the recent achievements of Northampton, who won the League Two title by 13 points, also helped to make up his mind.
The Cobblers staved off a winding-up petition midway through the season, as Thomas' takeover provided off-field stability.
Page continued: "I knew it was a fantastic club, especially with the achievements recently and what the club has been through. Credit has to be given to steer the ship in times like that.
"The players know they can compete at this level and be part of this. They not only won the division, they romped it. There's some great characters and quality on the pitch. They know they can compete at this level.
"Chris Wilder has done fantastic and deserves all the credit he has got from it. My job is now to manage that confidence so that we can compete at the next level."
Rob Page says Northampton chairman Kelvin Thomas was a key factor in him becoming the Cobblers' new manager.