Mainly because US investors are lazy investors:

1. The security is in USD (no change required)
2. The security trades at normal US trading hours (not with time delay due to time differential btw the US and the country of origin of the company)
3. It is treated as a local security in your tax declaration hence easier to declare to taxes from an individual investor’s perspective

https://en.m.wikipedia.org/wiki/American_depositary_receipt