In the good old days, rubber, tin and timber were exported from malaya through singapore.
That is why the rail from malaya ends in tanjung pagar, right next to the port.
All flights to UK/australia were from seletar airport. So tengku abdul rahman had to travel to singapore to meet the british to discuss about independence for malaya.
hence the airline that served malaya/singapore was MSA, malaya singapore airlines, based in singapore for all international flights, with domestic flights to KL, ipoh, penang etc.
Dr mahathir, and many others, had their higher education in singapore, as it was where university of malaya first started. It was only later that the second autonomous centre was set up in kuala lumpur.
In fact, singapore was the financial, shipping, airport, educational, film studio and trading centre for malaya. Even P Ramlee had to be in singapore to create his movie, even though he was from penang.
Only after seperation did malaya (malaysia) started to build its own airports, ports, airlines etc.