Popularity.
Max Factory/GSC is pretty infamous for not releasing complete sets of main characters for some series (Persona, Geass) whilst giving other series (Bakemonogatari) a mostly complete set save for one character, and giving some other series (Nanoha) releases for most every individual costume for the lead character(s) whilst other mains get shunned.
Then you get the Fate series case, where Saber is the only character of importance apparently, and we're lucky to get anything of other characters (For the record, there's been 7 sabers, re-issues of Armored and F/Z ver (I think Alter as well but I could very well be wrong)., Three of those were bundled with games, And we've gotten 5 other characters (Rin, Kerry, Irisviel, Rider, and Shirou), out of all of those only Rider got a re-issue, Shirou is [tornado fang]-all expensive and its still a mystery why they don't re-issue the main [tornado fang]ing character.)
And of course Miku is popular too, the other Vocaloids... not so much. But Miku is a [tornado fang]ing glutton in terms of merchandising whore power. So is Saber for that matter. Popularity works for their fans, but irritates the people who want something else but don't recieve because we're getting another Miku instead of *Insert character lots of people want to see here*.
The figure business is getting stale. Apparently Male figmas don't sell or something, and since I'm pretty much done watching new anime, I think I might be able to wind down purchases soon enough.
edit: oh boy I went on a rant about the wrong company.
Bandai's pretty bad at this too though, I bet half the cast will be Web Exclusive, thank god for bluefin!