The short answer is "yes."
You sir are correct.
"Register early and often."
The A150 website stated early on (from the creation of the website) there would be an artillery limit (event location space). And, MmM, has a by invitation only requirement, due to some authenticity standards the event has set and limits because of space (less than A150). I wouldn't say your were out of luck, just a little on behind a bunch of other artillery folks who paid attention. And both events, with some efforts to recreate history, only need a set amount of guns to fit into the scenarios, they aim to recreate. So, having anyone who wants to come play be able to well, just isn't historically accurate. (Never a problem with infantry)
Events are for the group, not for individuals. Folks just bought all of the product from the shelves before you got to the store.
Sometimes, that's the way it goes
Just a thought, maybe contacting the event organizers would do you more good than griping on an Internet board. Is there a "waiting list?" Sometimes folks find out they can't attend events for one reason or another. If you've already done that well, there's really nothing else you can do. Why not plan a trip to the "real" Antetiam battlefield instead?
"Grumpy" Dave T.
"Yet, despite these small numbers, Kernstown was one of the most decisive engagements of the war. The Confederates, Under Jackson, though soundly defeated, ultimately lost the most. As historian Bruce Catton observed: ‘The victory meant nothing at all, whereas the mere fact that the battle had been fought meant a great deal.’ Indeed, the ramifications of this odd little affair reached all the way to President Lincoln."