There are certain rules that must be followed to make entries into this ledger. If someone tries to make an entry in that ledger that does not follow the rules, the entry is rejected and does not become a part of the ledger. When BCH came into existence, it basically copied the BTC blockchain. It's almost like taking any file on your computer, copying it and then saving it with a different name. BCH also implemented new rules that went into effect on block 478,558. In order to add a new transaction to the BCH blockchain/ledger those new rules had to be followed from that point forward. These new rules violate the rules for the BTC blockchain, so they won't be added to the BTC blockchain. So now we have two distinct blockchains. Although they appear to be them same for the first 478,557 blocks, they are now completely different.