Prior to Drupal 8, Drupal used a module that was independent of Search API to integrate with Apache Solr. This module was called Apache Solr Search. However, it was decided that the Search API would become the module on which to base all search integrations for Drupal 8. A main contributor to both the Search API project and the earlier Apache Solr Search module for Drupal 7 is Nick Veenhof (also known as "Nick_vh" on Drupal.org). He explained the rationale behind making the Search API module a requirement of the new Search API Solr module:
"Drupal 8 has also helped in forcing us to rethink certain concepts and also live up to our promise to avoid the divide we had in Drupal 7 between the Apache Solr Module and Search API. Search API will be the sole provider of Apache Solr integration in Drupal 8. Search API in Drupal 8 ships with sane defaults (which you can enable or disable) that pre-configures the module so it can be used instantly."