Why Rome\'s allies were so loyal?

[Theodosius the Great]

What we call Northern Italy was the territory of various Gallic tribes, many of whom did ally with Hannibal.

Right, the river Rubicon demarcated Roman territory (i.e. Italia) from Gaul (i.e. Cisalpine Gaul). If any of those Gallic tribes were allies I'm sure they were of the lowest status. There were several varying degrees of alliances negotiated between the Romans and their subordinate client states especially in this period.

~Theo