I will repeat CyberAngel's statement and implication that "Only" 166 is not exactly a little. Even the Fate/ series, which started as a Visual Novel, has less. Though you may be talking about the amount of Touhou Visual Novels compared to other genres of fangames, which makes much more sense. 166 compared to the who-knows-how-many other Touhou fangames is small, especially for how long Touhou has existed and how many Doujin circles are specifically around for Touhou.
But to answer your question, Touhou Visual Novels may be rare because most Touhou Fans and Fangames usually focus on other types of game genres. Danmaku (Touhou's home genre), RPGs, Rogue-lites, and even Card games/Tabletop games are common. It doesn't help that Visual Novels, as CyberAngel said, are not a prolific genre and there isn't much of a hook that one can bring. It's a shame, really.