Torah Temimah cites Shevuos 6b.
And the reasoning is: since the Torah warned and punished regarding tumah, and obligated a korban regarding tumah — just as when it warned and punished regarding tumah it was only for tumas Mikdash and kodashim…
The parallel between the warning/punishment and the korban obligation keeps both within the same tumas Mikdash and kodashim scope.