Joseph,
First, make sure you have all the XSL files for the styles installed at
%appdata%\Microsoft\Bibliography\Style\
If any are missing, you can copy them from either
%programFiles%\Microsoft Office\root\Office16\Bibliography\Style\
or
%programfiles%\Microsoft Office\Office14\Bibliography\Style\
depending on your version of Word. After copying any file, be sure to close and reopen Word.
If you have all the files, and the other Styles are still not showing in the Style list box, then it may be because of the display language you are using. (Don't confuse display language with authoring language or proofing language.) A display language is used to display text on the ribbon and GUI, not in a document.
There is a bug that causes a style guide name not to appear when your display language is not one of the 44 key languages. There is only one key display language for English, and that is English (US). The other English-speaking countries such as English (UK), English (Canada), English (Australia), etc. are not key languages.
To change a display language, click on the Language icon in the Language group and select Language Preferences. Once the Word Options dialog box opens, the display language(s) are shown at the top in the section titled Office display language. The display language marked <preferred> is the one that Word is using. If it's not English (United States), then select the English US language and set it as Preferred, or click the Add a Language button to add it. Then reboot your PC.
If you don't want to change the display language, then you need to edit each XSL file that is not displayed in the Style dropdown list using Notepad (not Word). This link describes how to do that. Make sure you backup the files before editing them.
https://learn.microsoft.com/en-us/answers/questions/2337806/bibliography-styles-missing-in-word