The pattern nowdays seems to be to release a "sampler" dvd early then a season set latter on. If for some reason the sampler "tanks", they can simply cancel the season set.
I don't expect that to happen though, X-men is a good seller on video. The only time there's been a problem is with Warner Brothers and X-men Evo (stupid frakkers STILL won't release S4).
Yeah, why is that?
If you're talking about Evo...back when the show was cancelled, the way it was done raised a few eyebrows. No one but the producers and the studio know ALL the details, and none of them are talking (publicly). I've heard that the producers tried REALLY hard to get an S5, but they just got stonewalled.
I know that they HAD a S5 planned out (at least the arc) that supposedly dealt 1) with the fallout from Apocalypse's attack and 2) the Evo-vese's Phoenix and Dark Phoenix Saga. Some variant of DoFP might've been in the mix or it might not have (though the imagery in the final ep of S4 with all the Sentinels suggests they were thinking about it).
But the studio told them pretty much to "wrap it...right here and right NOW". It could be as simple as that Evo is a Marvel product, and WB is the home of DC. It also could have been that they had 4 seasons, and that's about as long as WB lets their shows run.