I would say it is how Mullins has previously campaigned his Gold Cup horses. GDC never ran over 3m as a novice and went Golden Miller, Al Boum Photo never won over 3m and though did go BANC the yard had Invitation Only and Kemboy for the shorter format (and Kemboy started the 2019 GC a shorter price than ABP). Djakadam went Golden Miller, not racing at 3m as a novice.
There are exceptions, Don Poli and Monkfish are two over the years, but in both cases they were horses who were strong stayers over hurdles. Don Poli only won at 2 1/2m over hurdles as it was the Martin Pipe and had been held up off the frenetic pace. Monkfish shortest distance he won at over hurdles was 2m 6f.
Arguably none of those mentioned went Arkle, but in all those times there was the option of 2 1/2m novice to run in, plus how many would of won a Supreme which on all known form, Ballyburn would of done?
In my mind a bigger BANC field will probably result in a stronger pace and more of a stamina test then we have generally seen in recent years and I just don't think Mullins wants to put his potential Gold Cup horses through that, not when you have a horse who has the pace for 2m.
Saying all this, I wouldn't be taking the 3/1 about Ballyburn for the Arkle just yet. If one or two others from the yard prove strong at 2m then could Ballyburn skip Cheltenham altogether? He also improved a great deal from his first run last year that he could hit a bigger price early on.
I've waffled enough...