Hey @Ron_Bowman ...

It's actually a pretty easy answer.  It has to do with the number of courses in the drop-down list.  Once that number reaches a certain limit, then you no longer see the drop-down and only get the text box to type in the course name/number you're looking for.  Check out this Guide:

How do I copy content from another Canvas course u... - Instructure Community (canvaslms.com)

Under the heading "Search for a Course", there is a single sentence that says:

"Note: If you have access to more than 100 courses, the search page only displays the Course Name field."

Does this help?