Running power on Series 5

I don't know why but Apple excluded this feature and all the new running metrics on the Series 5 (and below) for watchOS 9. There must be a hardware reason that Apple allows it on Series 6+.

• Running power: (Apple Watch SE and Apple Watch Series 6 and later) Apple Watch measures the effort you put into your run, displays the amount of power you’re expending (measured in watts), and helps you stay at a sustainable level.
• New running metrics: (Apple Watch SE and Apple Watch Series 6 and later) Running form metrics, including Stride Length, Ground Contact Time, and Vertical Oscillation, can be added as metrics in Workout Views.

From: Work out with Apple Watch - Apple Support

Only Apple knows the answer to that;