Why is cloud base estimation taught in Fahrenheit?
Why is cloud base estimation taught in Fahrenheit?

It's not, the source you are looking at is taught in Fahrenheit.

Jeppesen being an American company, I would assume that their teaching material uses units of measurement common in America.
There are many places which have the same formula in Celsius such as the wikipedia entry on Cloud Base among others.
I suspect that pretty much everybody looking to calculate this would understand the difference between Fahrenheit and Celsius and adjust accordingly. That's not to say that errors are impossible, but errors are always possible when calculating anything.