A significant fraction of the Go core engineers use proportional fonts when programming. On their screens, hard tabs are the only tabs that work. Spaces on proportional fonts are too tiny to be useful for moving code around.
It's not that Acme can't use monospaced fonts, it's that Rob/Russ/others don't want to use them. Proportional fonts are better fonts, so why not use them instead? One possible reason is that existing code formatting conventions assume that text is lined up in columns, but we have a tab key that magically lines things up: it's the whole job of the tab key. So, why not forget about space-based alignment, use the tab key for the job it was built to do, and get the advantage of using pretty fonts?