Former Real Betis coach Setien has signed a contract until June 2022 after Valverde was sacked on Monday despite winning back-to-back league titles and the 2018 Copa del Rey.
"We made a change now because we are halfway through the season and we needed some new momentum to face the final stretch of the season as we always have very big objectives," Bartomeu said.
"Even though the results were very good we knew that there was room for improvement in the team's play and we spoke to Valverde about this and in the last few days we had told him that the team needed to improve."
Barcelona romped to the league title in Valverde's first two seasons, finishing way ahead of nearest challengers Real Madrid and Atletico Madrid.
But the coach's reputation was severely damaged by twin shock eliminations from the Champions League to AS Roma and Liverpool.
The team's performances have declined this season and even though the Catalans are top of the table on goal difference, the board lost faith in Valverde after last week's defeat to Atletico Madrid in the Spanish Super Cup.
Bartomeu dismissed suggestions that the decision to sack Valverde was made hastily and with no clear successor.
Barca confirmed they tried to hire club great Xavi Hernandez, while Spanish media reports said they also approached Netherlands coach Ronald Koeman and former Tottenham Hotspur manager Mauricio Pochettino.
"I want to say that in the last few weeks and months we've been speaking to other coaches about the future, that's our obligation," Bartomeu said.
"The work had been done and so this was the right time to make a change."
Barca sporting director Eric Abidal said new coach Setien fit with the club's ideas on attacking football.
"We're very happy to present Quique, he has shown he has a clear football philosophy and is focused on the small details which reminds me of when I played for this club," said the former Barca defender, a two-time Champions League winner.
"We know he can achieve great things here." (Reporting by Richard Martin, editing by Pritha Sarkar)