Tunku Abdul Rahman revealed in the interview that he had only
wanted Sabah and Sarawak for Malaysia, but the British made it a
condition for him to take in Singapore as well. In layman terms,
the idea of merger was like a fried chicken meal. Imagine ordering
fried chicken, but the restaurant tells you that it has to come as
a set with coleslaw. So, it’s either fried chicken with coleslaw,
or no fried chicken at all. Singapore was the coleslaw, and Tunku
really just wanted fried chicken.

The Tunku : "If I had agreed to what Lee Kuan Yew wanted, he
would be a good boy for a while. Then he would go back to his old
ways of trying to create ill will between the Malays and the
Chinese through the Malaysian Malaysia campaign (ie. equality
for all Malaysian races, which would be unacceptable to the
Malaysian Malays and hence incite violence). I thought for a
long time, you know, and I was in close touch with Razak, that the
best way was to go on our separate ways but remain good friends. I
simply could no longer work with nor trust Lee Kuan Yew."

https://mothership.sg/2017/08/msias-first-pm-dissed-lky-shared-thoughts-on-spore-in-1980s-interview/