In those days, you were the guy who had to pay for all of the computer time expended. And if the computers had to work harder to solve problems, you would have to rent or buy more computer time. Moreover, yesterday's mainframes and itty bitty micros had little horsepower. But that's the time to create the equations in the first place. In actual use, BBDX is as speedy as other methods.
Linear frequency is simple and direct.
Everyone can understand how it works. For a given GRP level, you go to a simple equation and plug in the GRP figure. The result is the estimated frequency. From that, it's trivial to get your reach estimate. Linear frequency is simple.