Short answer: It is a restriction imposed by Oracle.
Not so short: If transaction control commands (commit/rollback) were allowed in database triggers, then other programs or stored procedures/functions would not have control over their own transactions. They would not have a way to decide when to commit or rollback their operations.
For example, if a program performs an insert into a database table (which has an after insert trigger with a commit, it that were allowed), and then the program realizes that other required operations cannot be completed, and thus the whole transaction has to be rolled back. What would happen ? The insert has already been committed, so it cannot be rolled back, and thus, the transaction atomicity has been lost.