I’m not sure this speculation is helpful. So obviously I will now add to it.
AK, Active and PM are based outside of London afaik. So they may not have been invited cos of wanting to cut down on travel / COVID exposure, which is something the organisers have said they were doing. Unless anyone knows different.
(We know there was disquiet about the Active banner in the past but they are also under new management now.)
ACG and Martin are based in London (Or have members here rather for ACG). So that is different. Probably there are more though. (Dunno if HSG applied for a stall or not).