https://people.com/tv/lifetime-announces-two-new-holiday-films/

Blending Christmas will feature former Brady Bunch stars Barry Williams, Christopher Knight, Mike Lookinland and Susan Olsen.

Any idea why two are missing from the reunion?

Well I must give the BB cast of characters credit, they sure know how to milk the show. Not saying this as a dig, I wish other shows did a little more of this too.

Any idea why two are missing from the reunion?

Maureen and/or Eve have typically held out on many BB projects, beginning with Eve in The Brady Bunch Hour. This has been an on-going thing for decades. I think it was this Q&A panel (https://www.youtube.com/watch?v=C5HX6HjRTpg) where fans said thank you to Susan, Chris, Barry, and Mike for doing so many BB events, saying something like, "We know you four always participate, and we so appreciate it."