Greet changes have taken place the last three years.这句为什么用take place而不用happen

(1)take place 一般指非偶然性事件的“发生”,即这种事件的发生一定有某种原因或事先的安排,例如：
Great changes have taken place in our hometown during the past ten years.
The Olympic Games of 2008 has taken place in Beijing.
(2)happen一般用于偶然或突发性事件,例如：
What happened to you?(一般不说：What did you happen?)
Maybe something unexpected happened.
I happened to see him on my way home.= It happened that I saw him on my way home.