because Barisan refuses to recognize nor formalize a two-party system.
"A shadow Cabinet such as those practised in the advanced democracies like U.K. are formalized structures recognized by the ruling coalition,"
"When a certain Bill is due to be debated, the relevant shadow ministry is invited to give its views and expected to challenge or support the Bill in Parliament. In other words, a two-party system is recognized as a norm. Can Najib look Malaysians in the eye and say that he and BN recognise a two-party system here?”
More of that where it came from: