Why treadmill motor speed controllers are not interchangeable?
This is an economic question, not an engineering question.

Excluding a government mandate, standardization only occurs when it is a benefit to the manufacturer. This most likely has not happened, and maybe never will. 

From a manufacturers point of view, standardization is often bad. For example, another company can make replacement modules cheaper and you lose business.

How standardization can be successful: companies A, B, C &amp; D get together and decide to standardize. Standard parts are now produced in greater quantities and costs go down. You take business away from companies E, F &amp; G.