Refer to the above link for those who still doubt that Sabah is officially the poorest in Malaysia. The reasons mentioned there appear to be reasonable.
Excuses such as Sabahan politicians are to be blamed because they are corrupted are not the real reason because these politicians are selected by West Malaysian politicians who are chosen, not because of their potential to develop Sabah but because they are those who are willing to surrender Sabah's rights to the West Malaysian government, i.e. the Federal Government.
First it was Petroleum, Customs Collection, Fisheries, and Vehicle Licensing, Telecom and Electricity, under Berjaya.
Under UMNO , projects were no longer managed by Sabah JKR but by federal government body or directly awarded to main contractors in West Malaysia. Even the appoinment of Chief Ministers and State government officers had to be approved by Kuala Lumpur that led to the rotating Chief Ministerships every 2 years.
Now KL wants water rights and the entire JKR department despite problems with JPPS and Federal JKR in West Malaysia, glaringly obvious full of mismanagements to the point that expensively new Federal showcase buildings are already leaking heavily.
If corruption were the reason, then there is no reason why Sabah was among the richest when it was the was corrupt under USNO in the 1970's but what is clear was that USNO had defended Sabah's rights without trampling on the livelihood of its citizens, and that was the time before Petroleum revenue was available.
What is worse is that situations in Sabah could be worse than in Rwanda, making Sabah the poorest in the world.