This really isn't that hard to understand, much the same way English was adopted as a lingua franca, Malaysia's Chinese population is itself heterogeneous and so at the national level the lingua franca of the Chinese nation, mandarin, is adopted over that of the disparate dialects that only had local superiority at city level.

A unified Chinese identity takes precedence over the local one, this is even more so when you are at the mercy of divide and conquer techniques. The only other alternative was Cantonese but then why would the Hokkien buy into that and vice versa? Educational resources are thin enough as it is.