I've no doubt it's all about cost, not that they don't think he'd sell or anything like that. If that were the case they wouldn't be making Mothra or Rodan either.
NECA likes to keep their figures in scale with others from the same series (as Mothra and Rodan are with Godzilla) and also look to re-use parts for other figures whenever possible. That'd mean Ghidorah would have to be huge, at least twice as big as their KotM Godzilla figure, and they probably wouldn't be able to offset the cost of such a figure by using some of its parts later on down the line, as they have with their Alien Queen for example. Plus the necessary price would probably push it into "super duper premium uber loadsamoney" territory, and potentially out of reach of people who just want to collect all four main monster figures at a non-super expensive price.
So yeah, disappointing for some no doubt, but I'm not entirely surprised by the decision.
"Absorb what is useful, discard what is not, add what is uniquely your own"
- Bruce Lee