To start with, comments are considered unimportant for tour examples as demonstrated eg by screen shots from here - if the question or answer have no comments, these posts are used in tour anyway (and I think this approach is correct). For the sake of completeness, there is a section in tour "Improve posts by editing or commenting" but it doesn't require presence of a comment either, because animation simply points an arrow to "add comment" link.
On the other hand, random comments that sometimes leak into example posts can set bad example - see eg the recent complaint at TWP meta (which inspired this feature request).
This is because (as opposed to questions and answers which can be voted down and close), there is no way for site community to help system find out whether particular comment is a poor fit for the tour. This means that examples may show inappropriate comments that just randomly gained a few popularity upvotes - all kinds of tangential jokes, rants, attempts at answers etc - which will serve as a very poor example for new users willing to learn from the tour page.
Nor would it be fair to expect of site community to pay same attention to comments as it does to questions and answers and spend efforts on monitoring and cleaning up all comments that may possibly leak into tour page if a system randomly picks some post.
The last but not the least, showing comments in examples tends to dilute the site purpose promised right at the top of the tour page: "Ask questions, get answers, no distractions" (this is particularly prominent in the example from TWP meta linked above, where comment is essentially an attempt at answer and is presented about as important as the post above it).