As a former victim of NGS (Nice Guy Syndrome), I can see a couple of things to think about here...
"Nice" is sometimes conflated with "non-assertive." Guys who are not assertive tend to not ask, or to mistake subtle encouragements.
Also, the "I couldn't date you, you're too nice," or it's close equivalents, is often used as a gentle let-down in place of other, possibly harsher reasons. Telling someone he's too nice is easier than saying he's not attractive or too nerdy or whatever. Sort of like saying someone has a nice personality rather than saying that they are plain.
Taken together, this creates the illusion that nice = dateless, when I think the reality is more complex.
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"Nice" is sometimes conflated with "non-assertive." Guys who are not assertive tend to not ask, or to mistake subtle encouragements.
Also, the "I couldn't date you, you're too nice," or it's close equivalents, is often used as a gentle let-down in place of other, possibly harsher reasons. Telling someone he's too nice is easier than saying he's not attractive or too nerdy or whatever. Sort of like saying someone has a nice personality rather than saying that they are plain.
Taken together, this creates the illusion that nice = dateless, when I think the reality is more complex.