Not all pros doing the Pro Challenge?

I saw a post either here on on Twitter recently that suggested that Giovanni isn't doing the Pro Challenge this year, and I noticed that Nikita was absent from the VT as well. So I'm wondering who isn't doing it this year, and why. Apparently 15 pros are doing it, so far we've had Katya, Karen, Vito, Luba, and Jowita (5), so that leaves Michelle, Neil, Nadiya, Nancy, Gorka, Graziano, Lauren, Kai, Carlos, Johannes, Dianne, and the aforementioned Giovanni and Nikita (13), so it seems like some of them are being left out, and I really want to know why if that's the case (I'll be disappointed if Nikita isn't doing it this year because I really wanted to see him take part).