If there's a new theme, it's more likely to be ignored because it's new. Most people don't want to go with something that isn't tried and tested to death. New individual cards will be accepted, yes, but not so much themes. Which is a shame, because they're trying with them (although I wish they'd just let Elemental Heroes die already - they're STILL not that great, even with a thousand related cards - my opinion only).
New cards for an older theme (such as Vehicroids) seem to be new for the sake of new. Stealth Union to me doesn't seem that great - it halves its ATK for no reason whatsoever, not to mention it's a smegger to get out. The other new cards are similar, I think - new, but not overly great. Without enough good cards, a theme just isn't going to get moving well enough to be liked.
That, and the Decks everyone is using are much quicker than the new themes. People like speed, and don't want to hang around waiting for their Six Samurai collection to grow enough to be useful. They like stand-alone cards much more than themes, which is why themes just aren't catching on.