User blog comment:Xilinoc/Ch. 562 - THE VILLAIN 2 (Mask De Masculine is a Mexican luchador in a German army)/@comment-1351982-20131218031243/@comment-2226235-20131218180430

It could also be because the Gotei 13 has rules against hollowfication. Urahara was exiled for experimenting with it and we don't know if they have changed the rules.