Just lucky, I guess.
Back in the old days when they had a call-in system we used to put a dozen people on the phones all calling at once. Now, the online registration system evens the playing field but kind of takes the fun out of it, in my opinion.
We are always open to any available date and also will accept a short trek if a full trek isn't available. Short treks used to be a separate call-in day and they got surprisingly few calls -- if we missed getting a regular trek we could pretty easily get a short one. Our 2003 treks was a short trek -- eight days on the trail instead of ten. I never had the feeling that there was anything missing; if anything, there's something to be said for being out a couple days less and having it end while you still want more.
In 2008, we requested any available slot, including short treks, and were given a short trek. But after the first payment deadline had passed and a few groups dropped out, Philmont contacted us and gave us the option to upgrade to a full trek, which we did. If I had it to do over again, I probably would have stayed with the short trek.
Now, with the computer-based registration system, you put in short trek requests at the same time as regular treks. I don't know if they still get fewer requests for short treks or if it has evened out, but if you really want a trek, make sure you indicate that you will take anything they have available.
Another option that I haven't had to use but would if we went more than a couple of years without getting a slot: Go with a council contingent. It is my understanding that any council can get a slot for a contingent trek -- they get priority over individual troops. You could go with a block of your scouts as part of the council contingent. Many years our council doesn't have enough interest (or adult organizers) to send a contingent crew, so if I asked, they're probably make me trek advisor and let me take as many kids from my troop as I wanted, as long as I saved some slots for other troops.
If you can't get them there any other way, you can send your Scouts with OA Trail Crew. A couple of scouts I know have done that they say it's a great experience (also cheaper than a regular trek). The only down side is you can go with them -- bad for you, but frankly most teenagers actually prefer it that way. You could always go spend a week at the Training Center and do a little hiking on your own. Valle Vidal is public land and incredibly beautiful.
Hope that helps.