Dev and Popey pretty much hit the nail on the head here. I can't say that I'm very pleased at a thinly veiled prodding of us to give up our community and join yours because a united community on a single forum will somehow be "better". Here's a (by no means exhaustive) list of potential obstacles:
1. AoKH is primarily a scenario design and modding community (and far and away the most advanced one in those spheres). AoCZone/Voobly is a multiplayer community, and has never really given a thought to the scenario design and modding side of the game (aside from a few attempts at balance patches). I'm unconvinced that AoCZone has the means or desire to create an environment where scenario designers and modders will feel as comfortable as they do here. AoKH has 16 years worth of forum threads, posts, articles, utilities, and a collection of over 12,000 files hosted on the Blacksmith here. Even more importantly, it has a tight-knit, helpful community of users that feels comfortable in a setting where they have all the resources they need.
2. AoKH's perception of Voobly is severely marred by the tendency of Voobly/its users to take content (scenarios, mod assets, etc) from AoKH without permission (or even crediting in many cases) and claim it as their own. While I'm sure that's as much to your chagrin as ours, it's nevertheless a major obstacle going forward if you're thinking about fostering a successful scenario design community.
3. AoCZone/Voobly (or perhaps more accurately, their communities) are pretty hostile towards the HD Edition of the game and its players. That's a big turn-off for a large number of users here, especially considering that several AoKH users are actively involved in the development of The Forgotten and The African Kingdoms expansions to the HD Edition.
4. As an add-on to the point above, several of the users from AoCZone/Voobly who have come here before have shown a rather disrespectful attitude. In the past the pattern has been to make a post advertising something, then leave for a while, then return to promote something/ask the designers here to produce content for Voobly, ad infinitum. That's a pretty one-sided relationship that didn't leave a good taste in the mouths of users here. One AoCZone user's short tenure here about a year ago was marked by incessant trolling and a post encouraging another user to commit suicide, which is obviously unacceptable on many levels.
5. As Dev previously alluded to, there are some benefits to being a smaller, but more tight-knit community. It's a face-to-face world here and designers like being able to receive quality feedback and encouragement from people they know and whose opinions they trust. The audience on other forums is somewhat faceless in comparison. Here at AoKH the scenario design and modding communities are well-established and the staff of the site put a lot of effort into improving things in that regard. On another forum where the focus is elsewhere, I'm unconvinced that these communities would be more than an obscure, hidden subsection of the forum. Additionally, AoKH (and other similar sites like AgeArena and HawkAoC) have 16 years of reputation as the premier scenario design and modding communities in the AoE2 community. Giving up that legacy and starting anew can only be detrimental to the survival of those spheres of the community at the level of excellence that is currently there.
That said, don't take any of the above personally - my (our) frustration is certainly not directed at you. I have a high level of respect for you as a person, Nai, and for the good work you have done for several years now. I would be glad to work with you in building stronger ties between our two communities, and frankly I think the "separate-but-connected" model is probably more beneficial for both communities.
On the topic of tournaments, it's probable that MS would only consider sponsoring tournaments for the HD Edition of the game and its expansions, considering that the HD Edition is the one receiving their official support. I can ask around and see what can be done on that front. On a related note, I'm sure you've been following the TEC event that ZeroEmpires has been hosting. It has been quite successful thus far and I'm sure it will continue to be in the future, especially with the "league" format it has adopted. Several of the users here have been following that event with much interest.
As for the divide between the HD and CD versions of the game, I find that rather unfortunate (and ridiculous to be honest). We all play and love the same game, regardless of the game version, so it makes me sad to see such high levels of misguided enmity on both sides. I think the best way forward on this front is the top-down approach - ensuring that the big names in each camp preach a message of acceptance and tolerance across the board. There will always be a few trolls here and there trying to stir up drama, but as long as the overwhelming trend is positive I think things will go in the right direction.
If you're interested in brainstorming some ideas to build stronger ties between AoKH and AoCZone, feel free to contact me by Skype (my details are in my profile). Cheers!