Why Apple watch 4 has not enabled ECG feature for the countries other than US?

The ECG function requires approval from the relevant regulators in the region, which takes time to acquire.

In the US, Apple worked with the Food and Drug Administration (FDA) for a number of years to receive De Novo classification for the ECG app and the irregular heart rhythm notification, making the features available over the counter. The ECG app and irregular rhythm notification are now CE marked and cleared in the European Economic Area.

https://www.apple.com/uk/newsroom/2019/03/ecg-app-and-irregular-rhythm-notification-on-apple-watch-available-today-across-europe-and-hong-kong/

The functionality is available outside the US, namely…

Austria, Belgium, Denmark, Finland, France, Germany, Greece, Guam, Hong Kong, Hungary, Ireland, Italy, Luxembourg, Netherlands, Norway, Portugal, Puerto Rico, Romania, Spain, Sweden, Switzerland, UK, US and US Virgin Islands

ECGs are considered medical devices by the FDA and other regulating bodies (WHO, EU, etc.) These require review of their performance, labeling and continued review of their safety in market, in order to be cleared for sale. The Apple watch is a FDA Class 2 device, and received clearance for sale and use as a medical device by the FDA in the US upon launch. Until the Apple Watch ECG feature gains clearance (or approval) in other countries, it may not be used in those other countries.

However, it has not received clearance in other countries. As of March 2019, the Apple Watch is cleared in Austria, Belgium, Denmark, Finland, France, Germany, Greece, Guam, Hong Kong, Hungary, Ireland, Italy, Luxembourg, Netherlands, Norway, Portugal, Puerto Rico, Romania, Spain, Sweden, Switzerland, UK, US and US Virgin Islands, and requires iPhone 5s or later on iOS 12.2.

Update: changing your region does not enable the ECG app. The hardware necessary is onboard all Apple Watch 4 devices, but not enabled, nor is the software required provided.