First, the article states that there's sand which can be safely taken from rivers, if it's taken from the right places.
Secondly, costs and prices matter (and are profoundly determinative in economics, especially absent effective regulatory mechanisms). The competition is already between opportunistic harvesting of sand from places in which it can result in tremendous harm, vs. managed and sustainable (even useful) dredging from elsewhere.
Thirdly, crushing rock itself takes energy (or transport to a location in which that energy is provided -- perhaps a river, or solar panels), plus capital equipment, which likely has a limited life (rock is tough stuff...). So: higher costs and complexity, and a much higher bar for entry, vs. locating a truck and a likely spot of riverbank from which to harvest sand.
Interesting refs for the used crushers, but I don't see cost or energy data.
An interesting comment from a company that sells used cone crushers: "Do you want equipment that is simple in design, easy to trouble-shoot and repair, and built to last? Or do you want equipment that uses high technology such as electronic sensors, computers, control consoles, and designed for high production but short useful life?"[2]
[1] https://www.youtube.com/watch?v=7gN7jcAx9Bg [2] http://www.usedconecrushers.com/Crushing__Plant_Design.php