As the church became more organized, the office of elder and bishop were collapsed into a single office. However, in the 1500s during the Reformation these two offices were separated and became distinct. In some churches several elders served within the congregation and were regarded as the spiritual leaders of the church. During this period many churches eliminated the office of bishop altogether in response to the Catholic church, believing instead that each church should be self-governing. Today there are a variety of organizational structures in the church.
Some churches have bishops but no elders.
Others have bishops, elders, and deacons all of which are ordained positions. And still others have no bishops because the elders and deacons oversee individual congregations completely. All of the them. The authors of the Bible didn't dictate a structure to be used universally in the church. Some churches had elders.