And I think that's cool and it's better he won't be the one explaining that any further (if there's a need for it).
Well, I personally prefer complete stories not some bit and pieces that could potentially be interesting, but never elaborate upon not because author can't do that, but because he doesn't give a damn about it.
The disappearance of the Belmonts is all you need to know for Order of Ecclesia's story to flow along.
Wait, what?
And what about some general information why it did happen? It is very important thing to the Castlevania (old) timeline and to the OOE storyline. OOE was a perfect opportunity to finally let that cat out of bag. I mean, story was directly related to the Belmonts dissappearance and it never was properly explained why it happened in the first place. They just...dissappeared and that's it. Sorry, but it is shitty storytelling in its purest, no matter how you look at it.
And idea that you don't need to know all details in the story, works only when said details are really insiginificant. Other wise you just accept that author didn't care enough to explain important plot points and treat as it's a normal thing. For that matter Belmonts disapperance is not an insignificant detail by any margin.