A rotameter requires no external power or fuel or any sort of energy, it uses only the inherent properties of the fluid, along with gravity, to measure flow rate.
It operates on 1st principle hence accurate and repeatability of the indications is very good.
A rotameter is also a relatively simple device that can be mass manufactured out of low cost materials, allowing for its widespread use. Pressure loss due to the rotameter is minimal and relatively constant because the area through the tapered tube increases with flow rate. This results in reduced pumping costs.