You can credit (or blame) James Watt for the formula. Funny how he is more know for the measure of electical power (you know, the watt) than for horsepower. He should have changed his name to James Horsepower.
Excellent article written in lay terms:
http://www.v8914.com/Horsepower-v-torque.htm
After reading this article, you will see that horsepower and torque must be identical at that RPM. Now if Watt had been observing a slower horse, say only 10 inches per second with that 550 lb weight, then I guess torque and HP would match at a slightly different RPM. Really was a little arbitrary.
Lynn