The Belgium international was snapped up by the Blues from Anderlecht in 2011, but has spent the past two season out on loan, first with West Brom and then at Everton.
The 21-year-old had been impressive during both stints, most notably with the Toffees last term as his 15 Premier League goals helped the side to a fifth-placed finish and a spot in the Europa League.
The Merseysiders have since secured the forward’s services on a permanent basis in a club record £28 million deal this week, and when asked why he would allow such a talented footballer to depart Stamford Bridge, Mourinho claimed it was down to his attitude towards his perceived entitlement in the squad.
“The thinking behind the move was the fact that Romelu was always very clear with us, in his mentality, in his approach, he was not highly motivated to come to a competitive situation at Chelsea,” said Mourinho.
“He wanted to play for Chelsea but he wanted to be clearly the first choice striker, which in a club of our dimension, it is very difficult to promise to a player.
“That reduced immediately his desire to come to us. Everton came in with an important offer. It is the reality of our club that we want to do things really well and to always be inside the rules (of the Financial Fair Play regulations).