The list is intentionally sorted by relevance based on fuzzy match. This is incompatible with an alphabetic search as that would potentially cut off the most relevant entry to outside the limited suggestions. This is similar to how tags are suggested in Phabricator, or how your text editor might suggest Quick Open file paths.

For example, searching for data suggests:

• Extension:GeoData
• Extension:JsonData
• Extension:DataTable2
• DataValues/DataValues

Instead of the first alphabetical substring match, which would be Extension:CommonsMetadata.

If you know what you're looking for, you can type additional letters or portions to quickly get to the intended entry. For example, vedit will suggest VisualEditor and pauth will suggest PluggableAuth among others.

To see a complete list, not limited to 20 relevance matches, click on the new "Complete repository list" link at the end of the menu (per T346074). This points to https://codesearch.wmcloud.org/things/?action=repos, where you can view (and using Ctrl-F, find) any repository in alphabetical order.