You got pascal, bar, psi, atm, torr, and all kinds of column heights of mercury, water and whatnot.
Why do we need all these different units to express pressure?
In: Physics
We have very few fields that have singular, consistent units. Those fields have one thing in common – they originated in the modern time. Radioactivity for example is measured the same everywhere, because it was pioneered by a narrow group and they made their mark on science.
Steam power was developed at similar times in the US, Prussia, Great Britain, France, Austria etc. and while they all copied each other, everyone has found their own special way to do things. Standardization was a national affair rather then an international one. If you were to buy a British made boiler or a Prussian one, you’d find different units to describe internal pressure, and different instrumentation to measure it. Those with sufficient access to mercury would use it in pressure gages, in places where it’s hard to find, you’d go with a water column.
Historical hick-hack is the short term answer.
It’s a wonder we only ended up with 3 temperature scales (I guess because Kelvin got it pretty much right immediately).
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