The former head coach was sacked in November having overseen relegation from the Championship and just two wins in 18 League One matches.
"I was surprised by the reaction after the dismissal of Sean. Our record was poor, I don't know how you can justify it," Lansdown told BBC Radio Bristol.
"He was here for 10 months and we won very few games and got relegated rather tamely. We didn't make an impact in League One, where we should have done, and we didn't play very good football. Need I say any more?
"The truth is the players on the pitch haven't produced and the manager in charge of the players didn't produce.
"We've changed all that now and we're looking to a new start."