How did stone tools change over time? why do you think these changes took place so slowly?

I would say its because no one really decided to change anything. No one during the old ages really knew how to do anything and only used what they had and what was closest to them.

I'm going to answer these questions in reverse order:

Changes took so slowly because back then there were nomads, there was no time to think, it was all about survival they used what they had and didn't really think much about it. No one is going to get creative while starving to death, but as nomads started to form communities and learned about agriculture they didn't have to worry about food that much and were able to think more. With multiple heads (the communities) and the time given all sorts of tools were created and changed.