Why is it easier for someone to perpetrate fraud using a journal entry than with a ledger?

The reason is that these more common transactions have a system of controls built up around them that is designed to detect a variety of issues. Conversely, there are fewer controls over journal entries, which makes it easier for someone to create a fraudulent transaction

Journal entry is easier for someone to perpetrate fraud because it is on the person's control what entry to make while the ledger is derived from the journal entry made.

 For the Business Transaction.

general ledger has control accounts 

For many reasons, one can be the access right is given to make journal entries, while mostly it is blocked on the ledger level. another reason maybe that any changes to the ledger can be traced easily through totals controls and can be highlighted when the trial balance shows imbalance or does not tally.