In a word – performance. Rendering was always the cheap way to finish a building. Unlike bricks, which were pre-made in quality controlled factory conditions, renders were made on site by all sorts of different people in variable conditions. Under the circumstances render got a bad name. [We could go on at length with the answer to this question, but it would take too long.]
Basically, the polymers in factory made renders give consistency and control of what is a powder material. All the applicator has to do is add water and machine mix. It is far easier than with normal sand/cement render to spot any misuse on site (not in line with the manufacturer’s recommendations).