I wonder why Malaysia didn't try harder to hold on to singapore.

It was because of the political economy of Malaya at that time, bear in mind that Malaya only constituted modern-day peninsular Malaysia, with Eastern Malaysia (Sabah and Sarawak) and the Strait Settlements (Singapore, Melaka and Penang) being under seperate British administrations.

The indirect rule practiced by the British in Malaya, meant that there was a largely indigenous feudal-monarchical class between the colonizers and the proletariat and peasants. Fast forward nearing independence day, the Malay-Muslim feudal classes in Malaya foresaw that they were going to take over an economy mainly owned by foreigners - namely, British colonialists and the Chinese and Indian middle classes. To guarantee further control of the country, they wanted Chinese-majority Singapore to be excluded from the federation. Sarawak and Sabah were then included to tip the demographic balance in the comprador Malay-Muslim class' favour. Back in those days the indigenous population only held a plural majority with no group having over 50% in Malaya and the Strait Settlements combined.

It was the progressive left forces that were for unification. With British help, reactionary forces defeated the Left Forces, which included not only (Chinese) Communist elements but Islamic Socialist, anti-colonial movements, and militant labour unions, the banner of internationalism vis-a-vis Singapore/Malaysia federation was defeated.

It should be known that the People's Action Party in Singapore, started as a social democratic party, which eventually betrayed the left-wing in their party (ie. the Communists) and then finally broke away from Malaysia. It was a mutually beneficial arrangement for both ruling classes in both societies. Singapore can weaponise it's city-state stature to become the economic centre of Southeast Asia like Hong Kong, while Malaysia's comprador Malay-Muslim feudal class was able to retain control of a backward racialised colonial political economy.

Furthermore, Malaysia under their first prime minister was Western allied. It was in the middle of the cold war under the throes of Western red scare propaganda depicting Chinese people as foreign communist agents. My 2nd part of the article I am writing will cover this more but the People's Republic of China had the foresight and took advantage of this situation to normalize relationships with Malaysia in 1974, which helped mend a lot of the Red Scare terror that the British implanted in Malaysia for decades. The same can't be said about the latent anti-Malay-Muslim rhetoric found in Singapore.

Another thing that was different was unlike Singapore, who can tout to graduate from the Third to First World, Malaysia never was able to escape from the Third World. A vast peasant population and large urban proletariat eventually meant that it would drift further left geopolitically on the world scale, while Singapore remained stuck in a badly put 1960s time capsule. This benefitted Singapore in some regards, it wouldn't have to deal with the difficult urban-rural and racialised contradictions found in present day Malaysia, but it also meant on a world-scale it was and continue to be geopolitically reactionary, in which the economic development of less populated Singapore was made at the expense of the more populated Southeast Asian periphery.