Most of the reason are pretty straightforward, and obvious to anybody who actually exerts some brain power to study the very simplistic formula.
1. The person who dreamed up the BMI said explicitly that it could not and should not be used to indicate the level of fatness in an individual.
2. It is scientifically nonsensical.
3. It is physiologically wrong.
4. It gets the logic wrong.
5. It’s bad statistics.
6. It is lying by scientific authority.
7. It suggests there are distinct categories of underweight, ideal, overweight and obese, with sharp boundaries that hinge on a decimal place.
8. It makes the more cynical members of society suspect that the medical insurance industry lobbies for the continued use of the BMI to keep their profits high.
9. Continued reliance on the BMI means doctors don’t feel the need to use one of the more scientifically sound methods that are available to measure obesity levels.
10. It embarrasses the U.S.
To put it politely, the BMI is junk science. The sad part is that your health insurance provider, and your doctor, probably use it.