Quick and simple answer - because it was more centrally located within the state than Iowa City, the original state capital. Iowa City was the Iowa Territory capital as well before Iowa became a state, and prior to Iowa City the capital (at least on a temporary basis) was Burlington (even further east). read more
Des Moines, Capitol City of Iowa With the influx of settlers to Iowa, citizens felt the state's capital should be moved to a more central location. read more