Why Do the Whole World Doesn't Subscribe to One Metric System?

It was way back in 1960 when International System of Units (SI System) was published. But I wonder why do we still use several systems of measurement!
S.I. system comprises of MKS units (metre-kilogram-second system of units) as compared to other system viz. CGS unit system (centimetre-gram-second system of units). There are other variations of measurement systems like United States customary units or its later modified version used in UK called imperial unit system.

In my country India, although SI system has been officially adopted but probably due to western influence of USA and more predominantly of British (who occupied India before its independence), several measurement units are in use. E.g. in medical science, weights of newborns is measured in pounds instead of kilograms. Height of an individual is generally measured in feet and inches and not in metres. Now a days, several multinational companies sell their products with the weight of ingredients labeled in ounces. Precious metals like gold is also measured in non-SI system.

Although road distances here are marked in kilometres, these are still referred to as mile-stones!

Isn't it very confusing?

Biggest blunder due to differences in measurement units

I don't know how many mistakes occur regularly due to difference in measurement units. To my knowledge, the biggest mistake in the history caused due to existence of different metric systems was the failure of Mars Climate Orbiter in 1999.

_{Wikipedia Image for the artist's rendering of Mars Climate Orbiter}
This robotic space probe mission was launched by NASA in 1998 to study planet Mars. If you don't know about it do read about it here.

On September 23, 1999, communication with the spacecraft was lost as the spacecraft went into orbital insertion, due to ground-based computer software which produced output in non-SI units of pound-force seconds (lbf·s) instead of the SI units of newton-seconds (N·s) specified in the contract between NASA and Lockheed. The spacecraft encountered Mars on a trajectory that brought it too close to the planet, and it was either destroyed in the atmosphere or re-entered heliocentric space after leaving Mars' atmosphere

I can't imagine any blunder that would cost this much! But we are still continuing with this mixed metric system of units and keep confusing ourselves.