To answer this, take a look at the map.
About 60% of Malaysia is located in Borneo, but it has about 20% of the population. That means that population density differs pretty dramatically. The ethnic dynamics also differ strikingly: Sabah is 80% bumiputera, but only a small fraction of that is Malay; Sarawak is 72% bumiputera, but only 23% Malay. The histories of Sabah and Sarawak are distinct from the Peninsula too. In fact, Malaysia was only formed when Sabah (formerly British North Borneo) and Sarawak (formerly the domain of the Brooke family) joined Malaya (along with Singapore for a short time). Never before had these territories been ruled together; historically, Malaya was associated with the Sultanate of Malacca, and Sabah and Sarawak with Brunei and Sulu.
As a consequence of all of this, party politics doesn’t work the same way. UMNO and MCA have branches in Sabah but not Sarawak—at least, not that I’m aware of, and Chinese politics both in and out of the BN often takes place in other parties that are predominantly but not exclusively Chinese (like the SUPP or the Sabah Progressive Party). Opposition politics also doesn’t work the same way, for many bumiputeras are not Muslims, meaning that PAS does not have the same constituency there.
All in all, you can’t look at party competition in the same way in these two states using the same lens. But these states are currently big BN strongholds, and looking forward, UMNO needs Sabah and Sarawak to retain power. The fact that these states are different does not mean that they are unimportant—just different.
One key issue is the BN’s longstanding policy of favoring bumiputera interests through the New Economic Policy and its successors. Because UMNO dominates the BN, and UMNO is Malay party at heart (despite being open to all bumiputeras), it is widely believed that non-Malay and non-Muslim bumiputeras don’t effectively benefit in the same way that Malays do. Watch this space for further discussion. UPDATE: See the New Mandala GE13 series post with Patau Rubis of Sarawak for more.