Hi Michel,
It may well be that it takes time for a new line to permeate the consciousness of collectors. Or it may be a fashion shift away from round watches (are we seeing other shapes becoming more popular?), or it could be, as you suggest, that AP are putting more effort into designing new pieces in this range, or advertising them more widely.
Some of the most collectible vintage pieces of today where not popular when they were first released, either because of size, or shape or design. Didn't the Royal Oak start off this way as well?
I was one who didn't take much notice of the Millenary pieces until I strapped one on my wrist a couple of years ago and was impressed with the 'fit'. Don't dismiss the size unless you have tried one.
Let's hope the trend with the Millenary line continues.
Regards
Andrew