Why Scorpio too common and Aquarius rare zodiac sign?

ive known Aquarius people before. One of my grand daughters is Aquarius...

Everyone has Scorpio somewhere in their natal chart, and everyone has Aquarius somewhere in their natal chart.

YOU are talking about the Sun Sign, which is easy to find so it is easy to make money selling your writing about it.  But it is only 2% of the entire chart, so it is not really what astrology is about.

You’re asking for something statistical where no statistics exist. Astrology completely ignores statistics, especially when they show astrology is false.

Depending on where you live, the winter season is a factor. Conception in January plus nine months...do the math.