For the last centuries, all volcanic eruptions in Iceland have occurred along a continuous belt. The Eastern Volcanic Zone has the width of 1.5` from east to west.
The reason, as anticipated here, is that the small scale mantle convection roll found at 120 km depth rotates excerts force from west to east on the tectonic plate. At the same time the area is pulled by the North American Tectonic Plate eastwards. These opposite forces create the rift zones and volcanic systems.
Volcanic eruptions will start in the other volcanic zones one day, and intrusions have recently been noticed on Reykjanes Peninsula and within Öraefajökull Volcano. One small eruption occurred in the ocean out of Reykjanes in 1926, as can be seen on the list of this webpage: https://en.vedur.is/earthquakes-and-volcanism/articles/nr/1874.