Was it announced that Britney would be on their song/video? I'm really asking cause I don't remember. It was 'When I Grow Up', right?
What I mean is that Pitbull really confirmed it and hyped up the collab, so it would be kinda weird releasing the song with some other female artist when everybody was expecting Britney. The backlash would be huge, and I don't see many artists or record labels disrespecting Britney that way. They replaced her on 'Hey Ma', but Britney was never confirmed I believe (it was all rumours) and it was said the movie producers chose to have Camila instead.
Also after hearing the kind words he said about Britney during POM (I mean I was there and heard him say how she has gone through so much and how her story of survival in the industry is an example for everyone), I think he would choose not to replace her out of respect.