Here's why:
* Appositive phrases are noun phrases that rename or explain a noun or pronoun that comes before them. They are set off by commas (or sometimes dashes).
* In this sentence, "tallest person in the class" renames "Ryan". It tells us more about who Ryan is.
The sentence can be rewritten to emphasize this:
> We asked Ryan, the tallest person in the class, to hang the banner in the cafeteria.