I want to know why all the Bitcoin has be in the Blockchain ?
Blockchain means chain of blocks,a block is like a contains that store or save bitcoin. Imagine when you small boxes or saves,put on top of each other to make a long block. This is how the blockchain is.
The block must be on each other to make the blockchain,the blockchain consists of every file of all information in a single block put together,this is possible by mining. Any transaction done in any of these blocks will be added to all the blocks put together as one (blockchain),so that any bitcoin spent in a transaction can not be used for another transaction.