Due to the design of the fan (radial fan), different cooling capacities and noise levels are achieved compared to axial fans. A radial fan is more efficient and quieter than an axial fan with the same cooling capacity. In order to achieve a good cooling performance with axial fans and to do so as quietly as possible, several axial fans are usually installed, but they are operated together at lower speeds so that it does not become too loud. Since the radial fan draws the air in from the side and then pushes it out 90° to the rear, a second fan makes no sense because otherwise the air flow would be interrupted. In addition, the second fan would be located exactly above the GPU chip, where no cold air would reach due to the centrifugal force, as it would be intercepted by the second fan and immediately pushed out the back.

If you are interested in more detail (and also want a better explanation), you can read the following article:
https://www.ventilator.de/bauarten-v...