It appears that Viking women dressed rather differently before they converted to Christianity:

"It's easy to imagine that the Christian church had certain reservations about clothing that accentuated the breasts in this way and, what's more, exposed the under shift in front," Larsson said. "It's also possible that this clothing was associated with pre-Christian rituals and was therefore forbidden" once Christianity became established.

Of course, considering the climate, it may also be likely they gave it up because Sweden is frikkin cold! But that's just me.