Why did Africa only leave the stone age when europeans conquered it?

I'm really sorry to see so much ignorance on display here. Although many Africans were indeed living in a primitive state when Europeans first arrived, there had been immense and sophisticated cultures and societies many hundreds of years earlier, in Zimbabwe, Benin, Mali to name only a few.

The ruins of Great Zimbabwe show a very considerable degree of craftsmanship, and their size indicates a large and thriving pre-mediaeval society.

When the early British explorers first entered Africa, they noted that the people didn't have a written language, didn't have any written history, no two-storied houses, didn't wear clothing in many cases, and hadn't even invented the wheel. 

The Muslim/Arab conquerors that came to Africa noted that they had to teach the Africans they conquered how to wear clothing. 

What they were doing was working so they didn’t change.

I think some parts of Subsaharan africa were in fact in the Bronze Age.

Oh, you mean like the jungles? Clearing jungle is not only among the most difficult back breaking work, it's compounded several times harder because the tse tse fly/malaria is so prevalent. I could expound but perhaps you're only looking for the following answer: ... because that's where black people are.

Because Europeans conquered it.  Just like any other civilization, they are fine the way they live until someone else comes along and shows them a better way to do things.

They didn't.  Sub-Saharan Africa adopted ironworking about the same time as Europe did.  What's distinctive is that they didn't have a bronze age, like much of the rest of the world did, but mostly moved right from the stone age to the iron age. 

well firstly it was actually beyond the stone age when they got there just like native americans were beyond the stone age when the colonists got to the americas, as  all human speices evolved about the same time from cave man to what you saw in the neiolythic period.

secondly if you are asking why their culture changed?

because it was forced to do so by the invaders

If you really want to know, instead of just spreading conflict, you should read a book called "Guns, Germs, and Steel" by Jared Diamond.  He answers that question about as well as it can be answered in that book.  It's not possible to give a simple explanation here.

Not even close to being true.  Read some books, learn the truth.