Elections for US Senate are state-wide. While the liberal bastions in MA went heavily dim-0-crat (e.g., Cambridge, Brookline, ...), many towns outside the 128/495 belts in central and western MA went for Brown.
Considering Barney Frank (Dim, Planet Unknown), on the other hand, is in the US House and is elected by only those in his Congressional district and not state-wide as was Brown.