As people are always looking for his old Grenadier barbarians or figures to match them, why doesn't he do these and the 15mm barbarians in 28mm?
Besides the reasons that Nick gave, maybe also:
1.) the old Grenadier barbarians (or at least most of them) are still available from eM4 and Mirliton, and those sculpts have stood the test of time very well (I think the oldest ones date back to 1991 or so)
2.) Sometimes sculptors just like to move on and do something new (even if it is just sculpting in a different scale)
Sculpting is an creative process, and thus requires plenty of inspiration and motivation, and to maintain that may require moving from a topic to another - sometimes this leads to rather obscure releases when a sculptor gets inspired by something odd, at other times it may lead to ranges left incomplete as the sculptor just loses the inspiration.
While I would not mind seeing Mr. Copplestone sculpting more 28 mm barbarians (which I would likely purchase), I think it is more important that he keeps on sculpting subjects that will help to maintain his own interest and productivity.