Do you know the current explanation for histone coding genes being so highly conserved?
No.
What sort of explanation are you looking for? I mean, the gross explanation is that the histones are conserved because of how critical they are to the function of the genome - mutations there are exceedingly likely to be deleterious, more so that in most other places. When one misstep means you're dead, it's not so surprising that there isn't a lot of variation.