Please correct me if I am wrong, but from what I observe, you can easily get 100-150 colours in a set of pencils, but if you go for sets of watercolors, there's rarely more than 24-36.
The answer is quite simple really: you can mix watercolours (and paints in general), whereas you can't easily mix coloured pencils.
For example: If I have two different blues - a light and a dark - and I want a blue that's halfway in between, with watercolours I can just take roughly equal quantities of each, mix them together and use that.
With pencils, I would have to apply both, and try and get the shading just right so that you end up with a medium blue rather than an uneven mess. Because this is more difficult, people prefer to have a wider range of colours, and so because there's a market for it, manufacturers sell this wider range.
Something else that may help to explain the difference is that watercolor painters can be quite picky about which paints they choose for mixing. Each watercolor paint is made from 1 or more pigments, which are some sort of substance that is used to make the color. For example, the color winsor yellow deep contains one pigment: PY65. In contrast, cadmium yellow contains two pigments: PY35 and PO20. Since watercolor is translucent, mixing colors can be a delicate process, and the more pigments that are mixed into a single color (regardless of how many different tubes of paint are used), the muddier the color gets. So, if you're buying a good quality watercolor paint set, it is likely to include mostly single-pigment colors and few pre-mixed (two-pigment, three-pigment, etc.), since single-pigment paints can be mixed more easily before becoming muddy. If the paint set included 100 different colors of paint, many of those colors would already contain three or four pigments, and would not mix or blend well with other colors.
(You can find more information about watercolor paints at pigments at https://www.handprint.com/HP/WCL/palette1.html)