Academy graduate Dawson, 23, had been an ever-present all season until head coach Jos Luhukay was axed on Friday.
Bullen, who was handed the reins following the Dutchman's dismissal, instead chose to pick the more experienced Westwood for Saturday's victory over Preston.
It was Westwood's first appearance for over 12 months after the Ireland international was frozen out by Luhukay.
Despite being pleased with the performance from the returning Westwood, Bullen says he wanted to personally explain his thinking to Dawson.
Bullen said: "I think Cameron has been one of the most consistent performers over the course of the season.
"I think he's done tremendously well and been unfairly criticised on socialÂ media because Keiren is such a fans' favourite.
"So instead of phoning Cammy or waiting until this morning, I decided to go around to his house last night and speak face-to-face.
"I think he deserved that as a matter of respect.
"Obviously he was disappointed and so he should be.
"It was a tough decision to make.
"He understood it was a difficult decision to make.
"But ultimately, Cammy and Joe Wildsmith are going to be at this football club for years to come and will play hundreds of games.
"Sometimes, they will disappointed and that's the manner of professional football - you're never going to play every game.
"It's the type of character-building you have to take on."