Because dishonest people took every cold spell and said "Ha, scientists said the globe was warming! This proves them wrong." even though anyone with a very basic comprehension would understand that increasing the amount of energy in the system through global mean temperature increases could indeed lead some areas to be occasionally cooler.

In any case, the IPCC has been using "climate change" since at least 1990;

https://www.ipcc.ch/report/ar1/wg1/

Because people didn't understand that an increase of overall temperature doesn't mean that the temperature increases everywhere.

It's about the overall temperature.

Therefore climate change.

Who is "they"? IPCC stands for Intergovernmental Panel on Climate Change and it has been called that since it was founded in 1988.

I'm no climate scientist but a few years ago when I went through some studies they seemed to universally refer to it as anthropogenic global warming (AGW).

IPCC is a political organization so they might be trying to take a more neutral tone?

When they realized they can unite "Global Cooling" from the 1970's with what replaced it: "Global Warming"! That way you are always right. If the world is getting hotter? Climate change! But if you were wrong and it's getting cooler? Still climate change!

Same reason Facebook changed to Meta. When a brand’s reputation has been sullied and mocked and delegitimized as a result, often the only option is a rebrand under a completely different name.