To be fair, I think the decision to continue a line or not comes primarily from sales. Marvel is an evergreen property, but one that very well could have died if not for the support of TRU and the increased popularity of the movies. Walking Dead is still going fairly strong because of the ongoing show, the general popularity of zombies, and again, TRU support. Almost all of the other Minimate licenses lend themselves to being one or two waves and done. TMNT could go long, depending on the legs of he show, and the greater focus on casual buyers and new distribution channels.
Godzilla, while it would be great to see that line carry on for boxset after boxset and give us every character in the library, just doesn't have the mass appeal. It's not really fair to say DST hasn't done their due diligence in trying to push Minimates; if they're not selling, they're not selling. Why fault them for trying in the first place?