Are we sure that was a personal reason? It could have been a 1000 different things.
I'm not sure, but I remember that it was Sia who said so.
And I'm pretty sure she would've worked with him already if she wanted to, she has worked with the others top producers ( Luke, Martin, Stargate, Timbaland ) and even her bff did a song with him. I'm sure it will happen eventually, though.
Probably we'll just get a small advertisement at the end of the video.
