It's not really a big mystery that cross-realm mailing has everything to do with all of the cross-realm technology installed in the game. So those who in their imaginations believe that this could have been done all along are very likely mistaken. And those who believe that they put it off for revenue reasons haven't come up with a decent rationale as to why they're doing this now. If it was so profitable and Blizzard was so greedy then it follows that they wouldn't bother with it at all.
Blizzard prioritizes their projects to suit themselves and there's a concept called critical chain where X and Y have to happen before you can do Z easily. This is likely part of that and while it may be news to some of you I don't think that cross-realm mailing of BOA's/Heirlooms is something that average players think about on a daily basis. Some do but I suspect that it's not a top-of-the-list concern for most others. Therefore it's not terribly surprising that Blizzard didn't drop everything to kowtow to that particular group of people.
It's like many other forum fallacies in which people believe that making a lot of noise about something is the same thing as demonstrating wide and broad-based support for the same thing. Not that common sense makes any difference at all to people who push their own agendas to the exclusion of everything else. It clearly doesn't. The nice thing about that though is that small groups of people pushing agendas are pretty easy to discount in the general scheme of things.