The most obvious reason is history. When you have used a system for as long as anyone can remember, a different system will seem strange and probably even wrong.
Also if e.g. gallon should change to Liter that would make the prices rather hard to comprehend, - at least for a while.
Also there is pride. Our system is better than your system.

But there is some movement and e.g. NASA is using the SI-system (meter for distance etc.) in most projects.
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