The real question should be, why weren't the British colonies of Sarawak and Sabah reunited back to Brunei.

Brunei was not officially a British colony, but a British protectorate. The British had full control over Brunei's foreign policy, but Brunei still retained some autonomy.

After WW2, Sarawak was handed to the British government by the last the Brooke Rajah and Sabah was a British economic colony.

Brunei is a Kingdom that has existed for almost 600 years, Malaysia is a new nation state.

Although a lot of anti-Bruneians might mock Brunei's weakness during the colonial period of the 19th century. A lot of people tend to forget that the Malayan states of West Malayan also feel to the British. Oh yeah, lets not forget that the mighty Kingdom of Burma also became a British colony.

After WW2, there was a plan to actually merge back Sarawak and Sabah back to Brunei, but thanks to some unknown idiot, that plan was abandoned in favor of the "Malayan Union" and then the "Malayan Federation".