为什么我在安装最新的reader DC的时候只能在C盘？

Hi 家辉杨

Sorry for the delay in response.

System default location to install application is C:drive. Adobe doesn't recommend you to change the default location for the application installation.

You may refer to the suggestion given in similar discussion here: How to install Acrobat Pro DC on a different drive? 

Note: If you do so, then any harm caused to the computer would not be entertain by Adobe.

Regards,

Akanchha