You might say that "work", aka resistance, is something that only happens at the point between where pos and negative meet each other. Both filaments are grounded but only the one that has positive meeting it from the other side will light. The other one just sits there.
By-the-way you could apply positive to one center contact and negative to the other, and both filaments would light, connecting through the ground in the base...if they were the same size. Because they are different, only the smaller one will light, the other doesn't get hot enough.
And I wouldn't know why you don't get continuity between the base and center contacts. You should.