。
Why didn't you come here sooner?
Literal
Why more-early here [to-に] come-not-past [explanatory-のだ].
A rhetorical reproach. The のだ ending on なんで~ない gives it the explanatory 'why is it that...' force — the speaker isn't asking for information so much as expressing frustrated surprise. のだ (rather than の or んだ) tips the register slightly more formal or declarative, the kind of thing you'd say in a novel or in a slightly stagey live delivery. Compare なんでもっと早く来なかったの (same basic meaning, softer) and なんでもっと早く来なかったんだ (rougher, spoken). The sentence implies that the speaker wishes the listener had arrived earlier and is now asking them to account for it.