There's two schools of thought on this:
1: Payware drives developers to make `safe` choices for the generation of maximum revenue by choosing only the `popular` models
I'd say there are sufficient examples of top-selling payware that was never asked for or thought of as `mass market` before being released to prove that theory specious - almost the entire Lionheart catalogue, Shockwave/A2A B377, the new Islander, The PMDG J41 etc. etc
2: The enthusiast market is a flight sim enthusiast market, not a `particular aircraft enthusiast` market. A good product, properly marketed, correctly priced and properly supported will provide it's own customer base.
L-39 Albatros - perfect example of a plane nobody knew we wanted, but now is the vanguard of comparison. Dunno about sales of the P-36, but that's another that creates its own market. Conversely, there are very popular aircraft that are modelled but which fail to set the sales charts on fire.
For the purposes of FSX, all you can conclude is that the `unpopular` aircraft are simply packages that don't sell, nothing more. Attempting to read any more into it than that on a broad-based theory simply isn't going to define either the reason for the popularity, nor the apparent failure. Both Crow and Cat have a point, but neither have an abacus, nor a crystal ball...