여서

Because it is (after a noun)

Gives a reason built on a noun: because it's such a big country, because he's the youngest, because it was with the members, because last entry is at nine. It is what 이어서 becomes in speech, so what comes in front of it is a noun or a noun-like word in every one of these lines. It does the same job as 라서, and the two are interchangeable here, with 라서 the more common spoken choice.