Here's a guess. Augsburg-Fortress tries not to publish things that don't sell; and since many congregations have abandoned the VBS materials it publishes....
Or, there is a backlog of materials already published, which can be made available.
The congregation that I serve began using Group VBS the very first year it was offered; and, in recent years, has not even wanted to make time to look at alternatives.
Why?
FIRST, everything is integrated with the lessons. It is 100%
teaching 100% of the time, and this is a very needed antidote
to the compartmentalization of our society. Other kits tend to
segregate "snack time" from "game time" and "craft time" with
no connection from one to the other, and, worse yet, little or no
connection to "lesson time". What does that teach? Well,
without intending to do so, it teaches that Christian learning,
prayer, and Bible study is just one little bitty slice of life safely
out of the way, not influencing anything else, and certainly not
the center and focus of all of life. Tony Compolo says that
either Jesus is Lord of all of life or He is Lord of none of it!
SECOND, every year one of the featured lessons is Jesus's
death and Resurrection. I simply can't understand that the
denomination kits and especially the so called "fundamentalist"
kits often overlook the very reason of our existence!
Of the kids who come to our VBS, probably 30-40% are
unchurched; another 20-30% are "C & E" (Christmas and
Easter) Christians at best, and the remainder are truly active in
this or another congregation. So many parents think that 5
sessions of VBS will make up for 52 Sundays of
neglect....that's why coming to cross and resurrection are
essential.