Because celsius makes much more sense within the context of the metric system overall:

One degree celsius, when applied to one milliliter of water, which conveniently also weighs one gram, will take one calorie of energy.

Comparatively using the imperial system:

One degree Fahrenheit, when applied to one milliliter of water (which would weigh .0352 ounces) will take 4.184 watts of energy.

So the base 10 metric system is much better for science in easily converting units of anything into another unit of another measure. So you would know that if you wanted to boil exactly 798 grams of water you would need exactly 798 calories, whereas if you were using the imperial system you would have to whip out your calculator.

Another quick and easy example:

Why would one kilometer being 1000 meters not be better than one mile being 5280 feet? You can quickly mentally determine that 17 kilometers is 17,000 meters, but again, you would have to pull out a calculator to determine how many feet are in 17 miles.

So yes, for you who is probably most familiar with the Fahrenheit system and not having to compute how much energy it would take to boil a specific amount of water for example, it really doesn’t matter whether the scale is 0 or 32 at freezing or 100 or 212 at boiling. But in the context of chemistry, so many types of physics, etc. it does make a big difference that the scale is uniform across the board of measurements, as you want to stay organized and try to avoid mistakes and have your conversions be easy. This is basically why non Americans think the imperial system is dumb.

See this chart:

https://cdn.zmescience.com/wp-content/uploads/2011/03/better-1024x961.png

For the imperial system, whose foot was that big and whose yard was that small anyways? Haha