The only argument I’ve seen officially (blue posted) regarding this stance is “so that they can accommodate region-based content” i.e. something only for EU, but not NA as example. Which is just a BS excuse because that has never happened in relation to forums anyway and in perspective resembles the most minuscule of reasons. Let’s say they have a stick announcement which is only relevant for EU, and not any other regions, just simply put [EU] in front of the sticky post. Sorted! How f’ing hard was that? Took poor peasant me a second to type. I should run for office, I seem to have solutions for the biggest of issues! /s.
I’m not joking around, last time they cared to respond to that question why the boards weren’t merged, their truthful answer that made any remote sense was region-based content/announcements/whatever. Its a joke, because I want a unified board as well, as we are treated as 2nd hand citizens here. We got our first CM last year, after HotS and its EU board had already been dead! I have seen regular posters from EU board since its early days (beta) simply migrate to US board as activity here was stale/dead.
The same thing for WoW EU board, got their first CM during WoD, uhm a decade too late.