Brentford co-director of football Phil Giles admitted that the Bees had sold Jota because of the financial constraints on the club.
Birmingham made the move to sign the Spaniard and the Blues have paid a club record fee to bring the wing wizard to St Andrews.
Jota was out of contract at the end of the season and, in order for the 26-year-old agree a new deal, the club would have had to shatter their wage structure which was something they weren't prepared to do.
Giles told the club's official website: "Jota has been a terrific player for Brentford. On behalf of the Club I'd like to thank him for everything he has done for us.
“This isn't the first time that we have sold one of our best players, and no doubt it won't be the last.
“When it becomes clear that a player has outgrown the financial constraints of the Club then it is usually best that they move on for a better contract and for a transfer fee that helps to keep us financially viable in this division.
“I'm confident that, as before, the rest of the squad will fill the gap and we will continue to progress together."
Dean Smith was, naturally, disappointed with the departure and believes the speculation had an impact on his performances this season.
He added: “We are all sorry to see Jota leave. He had an extremely productive period for us last season when he returned from his loan spell with Eibar.
“It was a pleasure to work with him and be involved with some of the special performances he produced – the goal against QPR quickly comes to mind.
“I believe the speculation around his future has not helped him repeat those performances this season but he has never shied away from playing and training.
“He was a positive influence around the place and we all wish him well for his future career”