BFPO v Malaysian GPO

Does anyone recall the reason why BFPO numbers weren't used in West Malaysia or Singapore?
Letters from UK were addressed as unit name, followed c/o GPO Malacca or c/o GPO Penang (and so on)

I'm guessing that it was a cosy arrangement that benefitted the local postal authorities.

From our side, it meant that we had to use local stamps which were usually quite attractive, and varied as each Malaysian State or Province had its own design.

Whilst we were in East Malaysia though, we used BFPO numbers (in the 6xx range as I recall) and of course the comparatively plainer British stamps.