well it's not really "just" called a robin. This is Erithacus rubecula, the European robin. Europeans (English-speaking ones anyway ) have a habit of just calling their birds by a single name because there are no confusion species, hence robin instead of European robin, wren instead of Eurasian (or winter) wren, magpie instead of Eurasian magpie. You get the idea.
Robin is a shortened form itself from the old familiar name (still in widespread use) "Robin redbreast". Roodborstje also means "red breast". Rubecula and probably Erithacus also refer to the red breast.
When English folk started travelling the world and laying claim to other peoples' property, they called a bunch of other usually-unrelated birds robins as well, hence the American robin which has a red breast but otherwise looks nothing like the European robin, and the Australasian robins (such as are found in New Zealand) which generally lack red breasts but are basically similar in body shape and behaviour.
