Welcome back!! :)
Pardon my cynical, but very honest point of view:
Perhaps it is because DCA is behind the times. Perhaps it is that a great, hardworking DCI corps member does not want to then go to DCA and be a peon working hard to only support a paid ringer who gets all the glory for standing in the pit for 90% of the show. DCI corps work very hard at every member contributing to the whole, this philosophy does not mean crap when the top DCA teams are judged how they are. In DCI the demand and its credit is member to member. In DCA who knows what it is? Maybe who has the nicest truck, or the skinnyest guard or the most money. It makes no sense.
So, You've got that, then you have corps who play the same fricken music for 40 years. Come on. DCI corps are progressive (like it or not, not saying it is good or bad, don't go there PLEASE) and DCA tends to stand for Dont Change Anything.
I am just being a devil's advocate here ( ^0^ ), trying to sport some thought...
while sitting here wishing I was still in corps. These are not my opinions, only ideas.
This old drummer really misses it.